first runup to 4000rpm on new 912ULS

Hmmmm! Just speculating;

Choke's are fully open?

Air filters in good condition?

Throttle cables - a weak point in Rotax engine control - do they give consistent movement throughout range (ie binding at any point)

"I've run it on the ground and balanced carbs/set idle".  Prolonged ground runs may result in sub optimal combustion leading to fouled plugs. 4000 RPM is not a full power condition (where any tendency to "foul" plugs is likely to be burnt off).

Fouled plugs could impact on larger than expected RPM drops.

Try swapping plugs around, rather than replacing. Your "normal" plugs will either continue to deliver clean electrodes or foul. If foul the problem is not the plugs.

Check that all plug caps are fitting securely.

High EGT's are usually an indication of  a "lean"  mixture - a contraindicator for fouling.

What sort of exhaust system are you using? (any potential restriction?)

Is this a new engine? If not how many hours?

brand new engine

gets were normal on both a and b

new plugs made no difference

when A turned off got a 1000 rpm drop

when b turned off get a 30 rpm drop

Alan

Swap the A and B ignition module connector plugs and repeat, determine if the problem moves to module B.

you’ve checked the ignition switch wiring and grounding yes?

will recheck the grounding

I have another ignition module and plan on swapping it out and see what happens

Alan