I was trimming a plane for level flight and I was using the wrong technique. I was adding little bits of ballast to the high side. My LHS guru showed me the right way.
Note that this is on a plane with a lot of wing surface and little wing loading.
He took a piece of tape about 1/2" wide and applied it to the upper surface of the wing, about 2/3 of the away out toward the tip. He folded the tape into a little "Spoiler" about a quarter inch wide. All it did was "spoil" the lift as the air flowed over that part of the wing. The plane was still off balance by weight, but he balanced the dynamic lift perfectly.

At the speed the rotor blades run, it seems logical that the tape flap would cause the problem you describe. I know, you already did the test, but again - the tape acting as a spoiler seems logical.

Still building the mental database.

Solo