Posting this one so you can add it to your gray matter if you ever encounter this malfunction.
I helped a friend with his recent build. The gun cycles fine, accurate, feeds, ejects no problem. When doing some shooting at the range during break in, he noted that it would not hold open after the last round was fired. Pull the charge handle back on an empty mag & bolt & carrier holds open. There was about 1/4" travel back after the bolt catch was pushed upward.
We did some testing this past weekend. We tried several different things to correct this issue.
Different buffer, buffer spring, mags, ammo, bolt catch no change. We switched uppers, with between two similar carbines, still no fix. I even cut a couple coils off an old buffer spring to give that a try.
Last thing we tried when all else was fruitless, we changed out his buffer tube with mine. Ta-da, fixed it. We were careful to pay attention to what the springs were doing as we disassembled it on the shooting bench...right out in the open.
Doing a stare and compare between the two buffer tubes it appears his tube is about 3/16" to 1/4" shorter.
Sadly, I do not recall were specifically I bought the stock assembly.
I traded him tubes as I do not care that it won't hold open. That particular lower is used as my wanderin' around prairie dog blaster & doesn't get run to empty very often.
Probably one of those ATI tubes ... there have been several threads about them being short/out of spec.
Glad you got it figured out.