Steps to reproduce on a beta cluster wiki:
- Ensure you don't have any users in your Special:Preferences echo blacklist (Muted users under Notifications).
- Go to Special:GlobalPreferences.
- Enable a global override for the blacklist and ensure that it's empty.
- Hit save.
Expected: The global blacklist is enabled but empty.
Actual: The global blacklist magically has a user inserted. The user is not in my local blacklist for any wikis.
Triaging this as high priority. Bug introduced as part of the work done in T198935.