diff --git a/jstests/noPassthrough/feature_compatibility_version/setfcv_curop_isolation.js b/jstests/noPassthrough/feature_compatibility_version/setfcv_curop_isolation.js
new file mode 100644
index 00000000000..86c910d49cf
--- /dev/null
+++ b/jstests/noPassthrough/feature_compatibility_version/setfcv_curop_isolation.js
@@ -0,0 +1,50 @@
+/**
+ * Verifies that a collMod issued by the FCV upgrade path does not leak its
+ * CurOp namespace onto the parent setFCV CurOp. Without the nested-CurOp
+ * shield, each sub-op's AutoStatsTracker mutates the parent CurOp's NSS via
+ * CurOp::enter(); with the shield, the parent's NSS stays at "admin.$cmd".
+ * Regression test for SERVER-122438.
+ */
+import {configureFailPoint} from "jstests/libs/fail_point_util.js";
+import {funWithArgs} from "jstests/libs/parallel_shell_helpers.js";
+import {ReplSetTest} from "jstests/libs/replsettest.js";
+
+const rst = new ReplSetTest({nodes: 1});
+rst.startSet();
+rst.initiate();
+
+const primary = rst.getPrimary();
+const adminDB = primary.getDB("admin");
+
+// Downgrade so the subsequent upgrade has real work to do (the
+// _cleanUpIndexCatalogMetadataOnUpgrade step iterates every collection via collMod).
+assert.commandWorked(adminDB.runCommand({setFeatureCompatibilityVersion: lastLTSFCV, confirm: true}));
+
+// Pause setFCV right after _upgradeServerMetadata completes.
+const fp = configureFailPoint(primary, "hangWhileUpgrading");
+
+const awaitSetFCV = startParallelShell(
+    funWithArgs(function (fcv) {
+        assert.commandWorked(db.getSiblingDB("admin").runCommand({setFeatureCompatibilityVersion: fcv, confirm: true}));
+    }, latestFCV),
+    primary.port,
+);
+
+try {
+    fp.wait();
+
+    const ops = adminDB
+        .aggregate([
+            {$currentOp: {allUsers: true, idleConnections: true}},
+            {$match: {"command.setFeatureCompatibilityVersion": {$exists: true}}},
+        ])
+        .toArray();
+    assert.eq(1, ops.length, `expected exactly one setFCV op in progress: ${tojson(ops)}`);
+
+    assert.eq("admin.$cmd", ops[0].ns, "setFCV CurOp namespace leaked from a collMod sub-operation: " + tojson(ops[0]));
+} finally {
+    fp.off();
+    awaitSetFCV();
+}
+
+rst.stopSet();
diff --git a/src/mongo/db/commands/set_feature_compatibility_version_steps/legacy_fcv_step.cpp b/src/mongo/db/commands/set_feature_compatibility_version_steps/legacy_fcv_step.cpp
index 639c4ad2211..826f37b6975 100644
--- a/src/mongo/db/commands/set_feature_compatibility_version_steps/legacy_fcv_step.cpp
+++ b/src/mongo/db/commands/set_feature_compatibility_version_steps/legacy_fcv_step.cpp
@@ -31,6 +31,7 @@
 #include "mongo/crypto/fle_crypto.h"
 #include "mongo/db/client.h"
 #include "mongo/db/commands/set_feature_compatibility_version_steps/fcv_step.h"
+#include "mongo/db/curop.h"
 #include "mongo/db/dbdirectclient.h"
 #include "mongo/db/dbhelpers.h"
 #include "mongo/db/generic_argument_util.h"
@@ -76,10 +77,13 @@
 #include "mongo/s/migration_blocking_operation/migration_blocking_operation_feature_flags_gen.h"
 #include "mongo/util/fail_point.h"
 
+#include <mutex>
+
 #define MONGO_LOGV2_DEFAULT_COMPONENT ::mongo::logv2::LogComponent::kDefault
 
 
 namespace mongo {
+namespace {
 
 MONGO_FAIL_POINT_DEFINE(setFCVPauseAfterReadingConfigDropPendingDBs);
 
@@ -283,6 +287,32 @@ void dropAuthoritativeDatabaseCollectionOnShards(OperationContext* opCtx) {
     }
 }
 
+// Helper function for running processCollModCommand() under a nested CurOp so the sub-operation's
+// namespace and raised dbProfileLevel (via AutoStatsTracker) don't leak onto the parent setFCV
+// CurOp. Without this, a slow setFCV can write profile entries to system.profile of
+// databases the user never enabled profiling on.
+//
+// See SERVER-85925 (original fix) / SERVER-122438 (regression).
+Status processCollModCommandWithNestedCurOp(OperationContext* opCtx,
+                                            const NamespaceStringOrUUID& nsOrUUID,
+                                            const CollMod& cmd,
+                                            BSONObjBuilder* result) {
+    auto parentCurOp = CurOp::get(opCtx);
+
+    // Pushed onto the CurOp stack; pops itself on scope exit via ~CurOp.
+    CurOp nestedCurOp;
+    nestedCurOp.push(opCtx);
+    {
+        std::lock_guard<Client> lk(*opCtx->getClient());
+        nestedCurOp.setGenericOpRequestDetails(
+            lk, cmd.getNamespace(), parentCurOp->getCommand(), cmd.toBSON(), NetworkOp::dbMsg);
+    }
+
+    return processCollModCommand(opCtx, nsOrUUID, cmd, nullptr, result);
+}
+
+}  // namespace
+
 class LegacyFCVStep : public mongo::FCVStep {
 public:
     static LegacyFCVStep* get(ServiceContext* serviceContext);
@@ -326,8 +356,8 @@ private:
                     collModCmd.set_removeLegacyTimeseriesBucketingParametersHaveChanged(true);
 
                     BSONObjBuilder responseBuilder;
-                    uassertStatusOK(processCollModCommand(
-                        opCtx, collection->ns(), collModCmd, nullptr, &responseBuilder));
+                    uassertStatusOK(processCollModCommandWithNestedCurOp(
+                        opCtx, collection->ns(), collModCmd, &responseBuilder));
                 },
                 [&](const Collection* collection) {
                     return collection->shouldRemoveLegacyTimeseriesBucketingParametersHaveChanged();
@@ -412,8 +442,8 @@ private:
             // deprecated catalog metadata and to correct any invalid value types previously
             // allowed in metadata.
             BSONObjBuilder responseBuilder;
-            uassertStatusOK(processCollModCommand(
-                opCtx, collection->ns(), CollMod{collection->ns()}, nullptr, &responseBuilder));
+            uassertStatusOK(processCollModCommandWithNestedCurOp(
+                opCtx, collection->ns(), CollMod{collection->ns()}, &responseBuilder));
             return true;
         });
     }
@@ -882,11 +912,8 @@ private:
                         // storage, issue the "collMod" command with none of the parameters
                         // set.
                         BSONObjBuilder responseBuilder;
-                        uassertStatusOK(processCollModCommand(opCtx,
-                                                              collection->ns(),
-                                                              CollMod{collection->ns()},
-                                                              nullptr,
-                                                              &responseBuilder));
+                        uassertStatusOK(processCollModCommandWithNestedCurOp(
+                            opCtx, collection->ns(), CollMod{collection->ns()}, &responseBuilder));
                     },
                     [&](const Collection* collection) {
                         // Only remove the catalog entry flag if it exists. It could've been
@@ -972,8 +999,8 @@ private:
                     BSONObjBuilder responseBuilder;
                     auto collMod = CollMod{collection->ns()};
                     collMod.setRecordIdsReplicated(false);
-                    uassertStatusOK(processCollModCommand(
-                        opCtx, collection->ns(), collMod, nullptr, &responseBuilder));
+                    uassertStatusOK(processCollModCommandWithNestedCurOp(
+                        opCtx, collection->ns(), collMod, &responseBuilder));
                 },
                 [&](const Collection* collection) { return collection->areRecordIdsReplicated(); });
         }