I had the same issues wit two RPIs and tried different PSUs as well as my lab bench PSU.
Only thing that fixed it was the official RPI power supply.
They’re only cheap so give one a try and at least that will identify the problem further for you.
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Did you see the post from @planetar about how to check the system state? It's outside of OctoPrint's control. Install RPi OS and you have the same command to check the problem.
Even with the warning gone, the Pi will be throttling it's performance and may run unstable. You just don't know anymore. Something is not working right somewhere.