Just a shot in the dark, but is it possible that you are too far advanced and are reaching peak pressure to soon? Just a slight alignment or measurement error could mess you up, and the rotary's resistance to detonation would mask the problem. A test would be to retard the ignition 1 degree, then disable the leading and trailing to compare the numbers to what you have now.
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Unless I am mistaken, spark too far advanced would diminish the effect of
the trailing plug and increase the effect of the leading plug.  What I am
seeing is the opposite.

Al