FWIW, here's what's going on with some (especially, but not exclusively) twin boom designs:

The high velocity jet efflux blasting underneath the stab creates a low pressure zone which then "sucks" the stab down, rotating the nose up. (For a demo, place a sheet of paper against your chin and just under your lip, holding the paper from the edge so that it goes up at first then curls away from as it sags over your fingers toward the ground. Blow across the top of the paper and you'll see it "lift" away fro the floor)

The higher the throttle setting, the more the stab is affected. Twin boom designs which have taller vertical stabs are not as affected because the stab is farther away from the jet blast. Shimming the front of the engine mount counters the problem by rotating the efflux away from the stab even as it uses the jet blast to hold the nose down.

They key is to find, through advice from trusted sources or through your own experiments, what amount of shimming results in the fewest/smallest trim changes over the widest range of speeds. It will vary slightly depending on your particular engine and CG, so don't be disappointed if you need to "tweak" your settings......

Hope this helped, Don.