I'm running an ARP 12.5" midlength 6.8 SPC barrel, Superlative Arms piston kit, YHM ULT 7.62 can. Lower is factory Aero SBR with PWS buffer tube and standard carbine buffer. Magazine used was 5 round CP products. Have not had any reliability concerns with this mag in my 16" rifle.
Rifle will not lock back on last round with any bullet weight with full gas. Suppressed, I'm getting slightly forward ejection. I'd say about 2 o'clock.
I experimented with the buffer weights and I ended up pulling all the weights out of the carbine buffer. Any weights, and it does not lock back.
It will now lock back w/SSA 115 grain ammo. Does not lock back with anything lighter.
Suggestions? I think it's strange that it appears overgassed with the suppressor, and yet has issues this badly unsuppressed.

Does your setip work with other mags besides that one giving you problems?

Any binding of the piston rod? I am doing a 6.8 piston set up now. Been tinkering with it for a bit. Found the piston was binding just a bit on the free float hand guard.

Open up the gas port and tune down the gas. 12.5" midlength barrel doesn't have much dwell time to build up pressure 

I agree. A carbine gas length seems more appropriate for a 12.5" barrel. Sounds like opening the port might be needed if there is no misalignment of the piston kit.