I confirm this is happening (XenForo 1.5.11).
No users are banned, deleted or merged, no any kind of import/export, no any messing with data via plugin or low level SQLs. Admin/Mod/User logs show nothing of that sort, but "(deleted member)" as user who liked content appears.
Cache rebuilding did not help.
What could be the reason? Can it be related to performance? Or liking, then unlinking and some kind of data desynchronization?