I agree with those suggesting that you have a non-firing plug.
Operating the engine at full power on the ground for short periods of time, and leaning to clear a fouled plug, is occasionally done without causing harm. Such practice has turned an unacceptable mag drop into acceptable for me on a few occasions.
You may notice that during operation on only one mag, the EGT is actually higher that it would be on both mags. I can't say that I know exactly why, though I've always suspected that the overly rich mixture is now being burned later is it exits the combustion chamber, and so the exhaust gasses have not cooled as much by the time that they reach the EGT probe.
Alos be aware that the precie positioning of the probe in the exhaust pipe can greatly affect it's accuracy. EGT manufacturers seem to recommend that the tip of the probe be in the center of the pipe.
I hope this helps