I just acquired a new T91A1 upper in 12.5 inch. The FSB is canted, nothing new there.
The lower is a PSA, stock is Magpul MOE on milspec tube, buffer is H2, spring is unknown carbine length. Trigger is Larue MBT.
I'm using a Law Tactical folding mech on the stock. This has never caused an issue with my Colt 11.5 upper so there must be something unique to the T91 bolt group I'm not familiar with.
When I first assembled the upper to the lower and static tested it more than once when I pulled the bolt back there was a binding of some sort that would stick the bolt to the rear. If I wiggled the upper to lower fit a bit the bolt would snap shut. Pull it a few more times, no binding, pulled again, binding. I wasn't sure what was up but it didn't happen anymore so I test fired it. The first round out of the mag stopped the bolt but it finally went closed on it's own after a second.
The gun ran without fail through the entire mag.
Today I went to zero the red dot, first round out of the mag stopped the bolt. No big deal, I popped the charging handle and it snapped shut. USGI, blue follower aluminum mags, btw.
I worked my way through the mag up to five rounds remaining when the bolt group bound up a little and caused a stoppage, round half way out of the mag. I popped the charging handle again and it snapped shut, fired the rest of the mag. I could tell the bolt was sluggish, I could feel it go home and lock into place after every shot.

What I don't know yet is why the bolt group is binding sometimes when the bolt cycles. Is there something about the way the chamfered bolt and the Law Tactical carrier extension work together that isn't compatible?
Or could it be that the Larue MBT hammer is catching the bolt weird or something?
At the moment I'm at a loss, looking for input.

Are there any markings in the folder extension that look shiny and New? Maybe mark the inside of the receiver extension with a crayon and see where crayon transfers to on the BCG? I don't have any direct experience with piston driven uppers but I would assume that since the piston is pushing the top of the carrier it may could make the rear of the carrier tilt downwards and bind?

I found the tolerances on my A1 to be extremely loose between the BCG and the upper receiver, loose enough that a Fostech Echo Sport trigger would not release the hammer. My lower was a known good LWRC lower, so I knew that wasn't the issue.

You may be able to solve your issue with a JP silent capture spring, as that should keep the BCG centered when cycling into the buffer tube.

Very interesting, I didn't know this thing existed. Thanks.

I had an issue with an out of spec buffer tube that jammed my bcg same way. Try and change out the buffer tube

I used one of Slash's gas piston buffers to keep the bolt carrier from dipping on my 16" T91. It makes taking the rifle down a bit of a hassle, but well worth the benefit.

Why not look at the Aero Enhanced buffer tube?

Thanks for the input and ideas, everyone. I've talked to Law Tactical and they gave me a plethora of things to check, which I'll do before I start buying more parts. Oddly enough they didn't say anything about the idea of using the silent capture recoil spring.
I'll update once I have the rifle in hand and check a few things out.