CheckReplDBHash can observe system.buckets.* in dbHash while a background FCV transition converts between legacy and viewless timeseries formats. Its direct collStats then fails with CommandNotSupportedOnLegacyTimeseriesBucketsNamespace (491), which is incorrectly reported as a dbhash mismatch. Retry the complete dbhash check on code 491; if the condition persists, the retry times out and still reports failure.