It is a larger lemmy issue. There is a bug logged for it already. There is some stuff you can do to fix it (which I have done) but issue is there were some email changes to lemmy between 0.18.2 a d 0.18.3 and there was no warning or anything logged when email broke so some admins are still unaware if users dont tell them somehow.
Another bug is logged for the fact there is no way to resend the verification email so some accounts are being left in limbo since the email gets lost and thats the end of the process at this time.
https://wiki.lemmyadmin.site
You can post it here and the explanation why it happens.