In my subsequent range testing I could not emulate the issue while trying both normal slide operations and potential errors in handling I could have caused.
I think this is an important factor. Subsequent testing. Particularly enough subsequent testing.
Mechanical faults tend to be repeatable events. Often, but not always at the same point in the operational cycle, but not always. But they almost always do repeat at some point.
I'm talking here about worn or damaged parts, primarily, but even something "stuck" due to debris of some kind, rarely only gets stuck just once.
All kinds of things are possible, including something that works "right" unless it is worked in just the right way to malfunction. But things like that generally always malfunction when operated in the way that creates the malfunction.
The key to recognizing what is going on is enough testing AFTER the malfunction to determine if it is a "one in a million" fluke or if it is something that happens over and over, or once in X number of times.
A defective sear engagement for example, the gun may not fire on its own every shot, but if it does it somewhere in every magazine, or even every hundred rounds, that shows it is a repeating problem.
Generally speaking if a gun malfunctions due to a mechanical issue, it will malfunction more than just once. IF, as in this case, one round fired, one time, and it never has done it again, with nothing done to the gun to change it, I'd tend to think it wasn't the gun that had the problem. Overly sensitive ammo, like duds and hangfires, shouldn't happen, with modern tech and production methods they are very rare, but they still happen. 99.99999% reliable is still not 100%.