Is taking off on single mag bad for engine

I inadvertently did not return the ignition switch to "BOTH" after a
preflght mag check cause: distraction, I was an idiot) the other day
departing in my Glasair with my partner. The engine is a 150hp
O-320-E2D. The engine only developed 1900 rpm (2200 is normal for us
on 2 mags with fixed pitch prop) and climbed poorly and ran rough.
After figuring it out in approximately 1 minute, I switch back to
"BOTH" and everything was fine. That was a few months ago. Our
recent annual showed some low compression in #3 (55) and the others
were in the mid 60s, down a few points from last year. Not what I
would like but the engine runs strong. My partner said that the
single mag takeoff could have caused "detonation" or "cylinder
glazing" or "a bad flame front in the cylinders" and ruined the
compression.
I don't think that at that low power setting in a 7:1 compression
engine anything bad could happen. I am not aware of anything bad that
could happen in general from single mag operation at any power
setting, other than rough performance. This would be the same thing
as having a fouled plug in a cylinder, and nothing bad happens then.
Does anyone have any other feelings about this phenomenon.