Problem with Apple filtering regular mail as Junk

For the past 2 weeks Apple mail (on my MacBook, my iPhone, iPad and in iCloud) has been filtering mail that is regular mail as Junk. Email from years past (going back to 2014) that has been dealt with, has beebn deleted etc. is now showing up in my Junk folders AND new mail, from senders that I regularly receive mail from, is ending up in Junk. It does not matter if I return it to the "inbox" it goes right back into Junk. I have talked to 2 people at Apple, one who knew even less than the first one, who recommended that I upgrade my operating system (which am reticent to do until this is sorted out) and deal with the 9K or so messages that are in my Junk folder. Back in April '24 I enlisted the help of a 3rd party app to filter Junk mail, but this is not due to their protocols, (have gone back and forth with them) nor would it explain the mail from 10 years ago suddenly reappearing. Anyone have any similar experience and solution?