could simply be bad pid tuning.

The bed, heater and thermistors will be different in the two printers, so you probably need to do some pid tuning.

thermal runaway is what i had last week.
Tip for you. Don't start heating thr hot end with pid tuning turned on, and then disable it while still running the heater.
The billowing clouds of white smoke that were vapourised pla - was my first clue I'd done something stupid :-)
Proper thermal runaway is usually indicated by smoke.

badly tuned pid or just the wrong settings is characterised by large temperature swings.
So you might have found that the bed temp hit 85c and then went back down to 40 and then back to 85 and so on.

In your case - probably just needs pid tuning for the new setup.
Did you flash the new board witht he old machine's firmware ?
If not that might be worth a try as it'll put the old pid setting back.

It might bugger other things up.
I have no idea, but - no it's not necessarily - or even likely to be - thermal runaway.