So your piston stopped at 20 BTDC and 24 ATDC? Just checking because if you checked at the small distance between 20 and 24 ATDC, it'd be a slipped balancer or bad pointer and about 20 degrees off
Regardless, still sounds like something funky with the balancer and pointer.
- I'd likely redo the test with the stop at a different depth.
- Make sure you are looking at the correct side of the sweep of the balancer to find middle
- When you are finished, put it at TDC and make sure both valves are closed (unless it's a very big cam or crazy ICL, they'll both be closed)
My gut tells me slipped balancer or wrong timing mark, just crossed up test, cam timing won't do what you are seeing.