I think I've found the culprit.
The gas block is loose. I was able to cant it to either side as far as it would go before the gas tube touched the forearm, just by using my wrist. I took pictures:
Untouched
And now, after I twisted it with my hands
Bottom view
Top
Side
Also, it appears that the gas block has slid forward of its intended position. That rim on the barrel should be flush against the back wall of the gas block, the way I remember it.
Looks like this one is going back to the shop from whence it came. I hope nothing else happens - this is the second time I've had problems with this upper receiver.
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