After a bit more testing I can definitely confirm that OS 3.5.X is bugged for certain games and apps with regards to the GPU clock bouncing up and down between 200Mhz and 1040Mhz. This is testing on an OLED deck though I assume it's the same for either.
The main thing that showed an issue for me was using Moonlight to stream from my gaming PC. Every 4 seconds or so there is a hitch/ micro stutter which shows itself in the frametime readout too. After tearing my hair out trying to figure out the cause I eventually tried pinning the GPU clock speed to 400Mhz and boom! Totally fixes the issue. It's the GPU clock speed bouncing up and down that was definitely causing the hitching, so it's definitely a bug!
Another weird thing - I tried updating the OS to the beta and preview channel but it goes totally wrong and weird. On reboot just shows a spinning blue steam logo on black background and you can't do anything but you can hear the game mode is there in the background when you navigate around. Eventually if you randomly hammer buttons you get a visible screen appear and are presented with a weird reduced menu with the option to come out of the beta channel and go back to stable, when all is fine again. Tried several times and got exact same pattern.
Never had anything like this on my LCD deck, is this just Steam OS beta and preview channels bugging out???