I just pulled a plug after running full throttle at 40-50 mph, and they look very lean, FWIW.

Then I went for a drive and richened the MPS 1/3 turn at a time, 3 times total. The problem never went away, but it was somewhat lessened.
After the last adjustment it was rich enough that the plugs were black. Obviously too rich, so I went back 2/3 of a turn.

So whatever is happening, a rich mixture will lessen the effects.
bye,

Csaba