There were some recent changes that added a bit more caution to unsyncing, specifically when dealing with blacklisted items. Since those items are not synced, we wanted to to add some protection when “unsyncing” them, so that there is a possibility of recovery, if needed.
You should see log entries for this in the sync activity log.
This has been the default behavior for a while now for any items that you have synced locally that are deleted by another user. Again, this is to provide a possible path for recovery. If another user deletes something from the cloud storage, the local trash may be the only place you could recover from.
In your scenario this doesn’t add any additional local storage use to user B, since the files were already local and are just being moved to the OS trash.