Electrical mystery--set out to fix 2 problems but fixed 3

What you're observing is not a fault for a portable generator, but rather an intentional design feature, and would not cause the GFCI to trip

I am aware of the issue and even have one of those plugs wired up. My UPS objected to that being used. But this does not explain how everything worked for over a year with no issues and when I checked outlets I got two lights. Or why two receptacle tester show two lights and one shows three lights.

So that theory does not fit the facts at hand.