the only thing i've ever ran into was somehow getting stuck at a multiple of the fps that gamescope was set to, for example the game rendering at 120fps when gamescope was set to lock at 60, 60fps if set to 30fps, etc. and only with RE engine games, like Street Fighter 6 or Resident Evil 4, i can get it to happen if i muck with the settings too much, gotta restart the entire steam deck to fix it
yea it's just really odd because it's not like I can only get 51fps, idk if it's bad juju to say but I'm running acnl on the emulator that shall not be named and the CPU is only using 1w and the GPU is only using 2-3 with little usage and no limits in gamemode
the only thing i will comment on is one of the first things i do when i image steam deck is i turn off discover overlay, unfortunately its in the base image, i can't uninstall it, rpm-ostree tells you its inactive, but the service still runs in the background. I have to open the settings and tick the box to make it stop running at startup. What I don't like about it is that it appears like it has a memory leak on it. Not saying this would cause your issues, but that's what's kind of a pain about being in game mode, you can't really see what's going on
if you're knowledgeable enough to know how, i recommend maybe keeping an ssh session open on another computer to your steam deck with htop or something similar running to monitor usage, maybe something spikes up in the background that hampers your fps?
but that's about the extent i can make a recommendation, might be a bit too broad, maybe someone else with more experience and perhaps someone whose seen a similar issue can maybe offer a more targeted troubleshooting step for you to try