I’m by no means a Merkel or German gun aficionado, but I recently worked on a late 90’s 147E. So I’ve been into a couple of these types of actions.

I’ve been thinking about your dilemma, it’s an odd thing that’s happening for sure. Has your gunsmith checked he position of the over hanging sear guide (the cone shaped rod that the sear spring rides on). Have you confirmed that the guide is in the correct position, is it straight or slightly bent….and that the coil spring moves (contracts and expands) smoothly? Obviously he’s removed the Sears and tumblers to work on the trigger pulls, so we can pretty much rule out burrs, bent, etc.
Highly doubtful the action is getting so hot with a few shots fired that heat would be an issue. Can you confirm that there’s not any gas leaks from pierced primers that is blowing into the action maybe causing an issue??
These actions are not overly complex. Your guns issues are really odd.
Have you actually seen the work he’s done to your sear and tumbler?