iPhone Mail and IMAP/POP3

You would think any email migration plan would have been designed to work seamlessly with the default Mail app on iPhones and iPads, but no.

After generating a dedicated app password, I created an IMAP connection to the designated Yahoo IMAP server, but it just will not download messages between 6/25/20 and 4/24/24. Messages before 6/25/20 are there going back to 2009, and messages after 4/24/24 up to and including new ones are there; there's just a hole between the two dates. Removing the account and recreating it to force a new sync yields the same result. The messages appear properly in webmail. What gives?

I even tried using POP3. It downloads a few thousand messages from current back to about 2018, then seems to get stuck in a perpetual download loop where it tries to grab the next 3000 messages, but fails a couple hundred messages in and starting over while deleting the messages that were successfully downloaded.

I am wondering if there is some sort of download limit being imposed by Yahoo or Apple (possibly Cox).