iPhone updates only new email; won't update from earlier today

My iPhone has a gap in my email (POP) between Sunday and today (Tuesday) 3:00 pm. When I make the Mail app check mail, it connects to server briefly to download NEW emails, then reports "Updated Just Now" -- but the gap in messages is still there. I know the emails are there. I just downloaded today's mail to my home iMac and all are there; I did the same with my iPad, everything downloaded fine. Furthermore, I can see them all there when I log into my mail server via webmail. It seems that my iPhone's Mail app is stuck and won't download anything earlier than 3:00 pm today.

I powered off the iPhone, and did two hard resets, but same problem. I've tried closing the app, I've toggled Airplane mode, I've tried on wifi only, tried on ATT LTE only -- nothing works.

This has been happening a lot the past month. The only way to fix is to delete and re-enter my email account, which I don't mind doing once or twice a year, but it gets old after the 20th time in the same month. (In 25 years of Macs, I've never had to delete and re-enter an email account, but seems everyone needs to do it periodically on iOS.)

Has anyone found a better fix than deleting the account?

Thank you!