My issue was fixed by going to IMAP settings and restyling over the greyed out forward slash after the IMAP server address.
I backed out of settings and shutdown the iPhone, and the restarted it a minute later. After the phone rebooted I went to mail and pulled down the mail to refresh it from the server. After a few seconds (which I think was the IMAP server resyncing with my mailbox, all my mail for the last few days came down. After two days it is behaving as it should with new mail appearing when it should. I note that this problem was not evident in my iPad which is iOS 17.x and not on my M2 MacBook Pro, nor on my partner’s iPad with iOS 18.1. I can only conclude that Apple can solve the mystery of this and the other issues with email that can’t be solved with this fix.