The only thing I had to change was in the second screen, in which selected All instead of 30 Days.
For all of this discussion, though, my concern is that this is just a general issue that can't be fixed by you or I. For example, I came across this article today:
https://www.androidauthority.com/delayed-gmail-notifications-1052389/ . If so, that's really bad and something that needs to be fixed ASAP. This article is from November, so that's already too long as far as I'm concerned. For all the things they can do, a smartphone is, to me at least, still first and foremost a communications device!