Edit: nevermind 
after another half hour or so, one of the core temperatures spiked to 100C, and now all cores are stuck throttled down to 400MHz again.
Thought I’d follow up here as well: after going through a ton of troubleshooting steps, Framework Support finally sent me a new mainboard, and that seems to have solved this issue (Excessive CPU thermal(?) throttling on 12th-gen - #12 by kelnos for details).