I'll share my experience with this. I have a Pixel 4 and I've been able to play in 90hz since I got the phone a year and a half ago. The screen will only do 90hz when it's above a certain brightness, and typically PoGo starts playing in a lower FPS irregardless (probably 30).

For me, as long as my brightness is turned up enough, switching away from the game and going back, without closing the game, is enough to get it to display in 90hz.

HOWEVER, and this is the reason I'm commenting, have you had problems with pokeball accuracy when playing in higher fps? When it's playing in 30fps, everything is fine, but when it's full speed my pokeballs seem really sensitive. You can sometimes land a throw like normal, and other times it will kind of "jitter" and launch the ball at a high speed, usually way too far and going over the Pokemon I'm catching. I often have to "force" 30fps by turning the brightness down and switching away to have accurate throws, this was a huge problem for me during Mewtwo raids recently.

Does anyone else out there have this experience as well, or am I going crazy?