Drop in RPM magneto test

On my Rotax 912 UL, I'm experiencing a 1000 rpm drop in RPM when I perform a magneto test at 4000 RPM. Could someone please help me identify the cause and fix it? .

I've tried swapping ignition modules with ones from another aircraft that doesn't have this problem. I tested it with the swapped modules, but the RPM drop persists.

Thanks