This is a known bug. Mentions only exist as updates, and updates get erased after about 3 months or so. So when a record with a mention gets saved, iNat looks to see if that user has already been updated about it, and if there’s no update, it makes a new one.
As to whether it’ll get fixed in the notifications revamp, I’m not sure. A fix would be to make mentions persist permanently in our database, so that would create a lot of new records.