That would happen if a piece of crud acts as a glowplug. Very bad.
If the engine misfires on one mag but runs smooth on both then its most likely plugs or harness leads. Try swapping the plugs top to bottom and see if the problem migrates to the other mag. If the problem is still there then swap the capacitors in the magneto cover (I’ve had severe misfiring caused by a failing capacitor) once again to see if the problem migrates.
Sure; my concern would be that something may have come off inside the magneto, and is rattling about. After any event like this I would inspect everything to do with the ignition system before further flight or even before starting the engine again.
I agree, doing a lot of testing that involves starting the engine is too risky. The misfires are extremely violent.
The engine ran perfectly well on both mags. The first turn of the key provoked the backfiring. I turned the key quickly to the other mag and the engine was running ok but there was no mag drop anymore from both to the one that was ok.
To clean the plugs I ran the engine on full power leaning to just below max TIT but this did nothing. When checking the mag again the backfiring was as violent as before. I am pretty sure it is the mag but I hope my trying a few times has not provoked additional damage.
That plug cleaning method often fails to work.
I have known people who took off anyway, if it was a single duff plug… 
It does always clear in flight, apparently.
But this sounds like more than one duff plug.
Sounds like one magnet fires late. When both are selected the burn starts when it should and when the late magnet fires the burn is already in progress. when run on the late magnet the burn peaks when the exhaust valve opens. Impulse coupling counter weight stuck, or other internal mechanical failure affecting timing.
