jstests/core/query/comment_field.js fails intermittently in the fcv_upgrade_downgrade_replica_sets_jscore_passthrough suite because the background FCVUpgradeDowngradeInBackground hook can interfere with profiler writes. The test uses profiling at level 1 with a failpoint to make operations slow enough to be profiled. When setFeatureCompatibilityVersion runs concurrently, its lock acquisition can cause the profiler write to system.profile to silently fail, resulting in 0 profiler entries when the test expects 1.

This is a test/environment interaction issue, not a server code bug. The comment propagation from aggregate to getMore works correctly (confirmed by $currentOp).

See BF-42539 for the full analysis.

Fix: Add the cannot_run_during_upgrade_downgrade tag to comment_field.js. This tag is already used by the FCV upgrade/downgrade suites to exclude incompatible tests.