There was a recent change to the way TL3 requirements are handled.
https://github.com/discourse/discourse/commit/c45574c9e72ad0235bbdd0cada5933df8c9f306d
PMs are now excluded when we check for TL3 requirements. I guess the 6 users you mentioned have a lot of PMs which made them qualify for TL3 even though their public participation was not enough.
If that’s the case, those users need to participate more in public topics to regain their TL3 status.