i also have this issue, my iphone 12 pro max has often dropped frame rates when opening apps and running some apps and whenever, i have tried contacting apple and run diagnostics with it they said my phone was okay but still im experiencing dropped frame rates, i got the dropping frame rates since i got my phone and i thought this was normal but it kept happenig on my iphone for 3 months so i tried contacting apple yesterday and they will give me a call today i hope i get my iphone replaced as it is so annoying