The brass case is softer than the steel case. The brass case expands/seals tighter than a steel case.
Upon firing, the pressure/heat expands the brass case tighter against the chamber forming a great seal. Steel case essentially does the same, but not to the same degree.
As pressure/temp drops, the seal breaks, facilitating extraction.
If the bolt is trying to unlock/extract before the brass case has 'broken seal' the extractor hook can jump the rim, leaving the fired case in or partially in the chamber.
The bolt continues rearward, picks up the next round from the magazine, and pushes it forward toward the chamber, except the chamber is not empty.
Classic double-feed type of jam.
An adjustable gas block is one way of addressing the situation, by dialing down the volume of gas.. A heavy buffer will usually work too, as it retards unlocking/extraction.
I've been through this with one of my 5.56. An H3 buffer sorted it out nicely. Now I'm running (yes I said 'running'...lol) H3 buffers in all my carbine lowers, and everything runs well. I'd probably need the standard buffer in the arctic.
I'm no expert, but I believe that this is what's happening. Especially since the OP indicates that lockback is happening with a single round.
