WD40 didn't fix the situation. Nor did other products. I don't see anything that might have fallen into the mechanism. The latch will work if you insert the buckle and move the release button a little. Definitely a head scratcher. Searching for a replacement gives me the impression this is not a new problem, since there are many replacements on the market. Looks like I need to bite the bullet and replace the latch with a OEM replacement. A YouTube video makes it look fairly easy to replace.
only thing I can add is the newer the gm product the more cheaply it is made
lot's of seat belts on the market always has been, has nothing to do with defects and more about wear or repair after a accident, restoration, etc
the retention springs sometime need work there are company's that just do that exclusively.
the belt's get dirty looking or sometimes frayed, so there is always a market
the plastic housings crack or pop apart, so on.....