I experience this as well after upgrading to Catalina. I thought it was my fault at first, caused by some sort of key combo. It only happens for me when in full screen with another app. I use Mail with a GSuite account as well.
I am still not on Catalina yet and it started for me ca. 6-9 months ago. Perhaps with some Mojave update. (unsure as no big changes occurred and it took some time to see being a systematic trouble) I am more and more turning off system updates (not only Mac) as those bring more trouble than good. Being a very late adopter pays off. (update to Catalina is still not in the middle term plans)
Admittedly I am not a developer, and none of the software I’ve wanted to run has required a more recent version.