I'm having the same problem. It's on the Fido echo where people can search for files. For some reason, the most recent message always shows as new/unread.
Interesting! This is exactly where I'm having the issue as well.
As sort of a work-around. I just discovered that if I delete the newest
message from that echo, then do a message pack, that seems to have done the trick. (Perhaps that's what I did the first time and just had forgotten.)
--- Mystic BBS v1.12 A46 2020/08/26 (Windows/32)
* Origin: CaNerDUH BBS (21:4/151)