My wife and I were astonished when we noticed that her phone started exhibiting the same behavior this weekend (hmm, mine started misbehaving on a weekend).
This time, I started doing a force stop on apps, and not restarting them. I did this about 6 at a time, then walk out of the house far enough to drop the network & then back.
I reached the point where the phone would reconnect, and looked at the list of apps I had stopped that time. I restarted Gmail & FitBit, and tried again. No reconnection. I removed FitBit, and the phone would reconnect.
This is really odd, as I uninstalled FitBit on my phone when I was having the issue, and that did not help.
I have FitBit running on my phone now, and it works fine. I'll probably re-install FitBit on hers. I should have noted what version of FitBit was on her phone. I have 2.9.1
So, if you're having this problem, and have FitBit, uninstall it first, reboot your phone, and see if that is the issue. Reinstall FitBit if you want, and try again.