If you read the tl3 requirements tito see that they have to continue reading some percentage of messages over the past length of time. Unlike tl2 you can fall out of tl3.
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.