Mine worked the first time until I disassembled it and reassembled it. I figured out the same thing you did, but I messed with it a bunch (including not installing the recoil spring assembly just in case it was the barrel lug and locking block–now obviously not it)
I can see how the beta testers could have missed it because it got stuck 4 out of 5 times at first (my first successful try was a fluke), then I was able to figure out a technique to get it seated correctly every time. But even if seated correctly, if you just nudge the guide rod a teensy bit you’d hear it slip and it would get stuck, lol
It’s good it’s fixed now but the struggle to figure out what was going on was actually pretty useful to me, I’m now way more familiar with the internal workings than I was at the start (not the first time this kind of thing happened in 3D2A, of course)
I won’t be able to test mine for a while, since I haven’t ordered the AR FCG yet, but the one I put together was a personal remix that uses the DB9 mags (since I already had a lot of them)
Handguard is also modified to my liking. Applied my own guide rod channel fix before the official one came out. Made the charging handle end plate shorter and right side only so that the brace could fold more flush. The itty bitty top rail is also different (lower height, printed separately, attached with an M5x16, not compatible with the full length rail addon)
The mag well was loosey goosey with the DB9 mags, but I tightened it up. One source of looseness was that the top part behind the locking block had too wide of an opening that the mag could tilt forward into. That might not have been a problem with the sten mag version, but since I had to open up the lower rear of the mag well for the mag catch lever, it had room to tilt backwards there.
The parts I filled in were for the trigger bar and slide catch, which we are obviously not using here. Had to make sure there was clearance for the barrel to tilt, and it’s good there for sure.
Mag play feels just right, and it feeds snap caps reliably at least
I changed the ergos, but fundamentally it’s still the original design, so I’m waiting for good results from your firing tests
I might make a further remix to add a dust cover or a full length top rail, but every attempt so far makes it either look too bulky, too weak, and usually an awkward way to attach it