P-mags can and do interact with the bolt-hold-open in a TAR-21! There's been some back and forth over whether or not this is possible. It is. You can see footage here. I realize the movement you see in this video is small, but its seems to explain previous problems.

What I believe happened to my rifle is this:
Shooting with standard p-mags caused my bolt hold open device to partially raise during operation, which in turn caused pre-mature wear on my bolt and bolt hold open.
This wear on the bolt may have been the source of some of the failure to feed jams, and bolt hold open failures I experienced at the start of the spring.
Since modifying my magazines and replacing my bolt, I have not had any further issues with the rifle.
The p-mag, e-mag, 20 rd p-mag all have the same rounded rear face. The new M3 magazine from magpul is specifically designed to address this issue in non AR-15 platforms like the SCAR and TAR-21.

Hopefully this video will help others from running into the same problems.