Searching a number of other Apple forums, this is an iOS bug and it has appeared in versions prior, but been fixed prior to it being released generally.
for myself, I went to my mail server, repacked all the certs it uses, validated using several online tests and yet iOS and iPadOS 18 still fail to fetch imap mail and the server log reveals that it’s an issue with those clients.
it a bit irritating that such a thing, after you see it’s been a known bug in the stack for the past two years - would get out in the general release.
it’s more irritating when you see what is planned for the mail client in the upcoming Apple intelligence release. There is no workaround for this bug if you want to use the features of the $1500 iPhone pro 16 and latest $2000 iPad pros.