My WAG would be magazines as well.

Point; BCM advises H3 Buffer with Supers. I run H3's in 4 different 300 BLK's, shoot nothing but handloaded 110 gr BlackTip at 2400 FPS with no issues.
I'm not familiar the JP heavy spring.

If it were me I would source some different mags (I run Brownell's aluminum in 300 BLK) and throw a conventional H3 buffer and Sprinco Blue or Green and test again.
I'll add I would try one component change at a time to narrow down the issue.