Mail widget does not update

My mail widget does not update. I always see the same 2 unread mails, although I already read and deleted them.

Same issue as this closed thread -- Mail widget does not update - Apple Community

Rebooting my phone works, but I am tired of rebooting multiple times per day. It makes the widget useless. Is there a better fix?

And yes, my OS is up to date. This issue has persisted for months, through many updates. And many, many reboots...