Even if you've played it 6 times, there would still be like a 26% chance you never encountered it if the chance of getting it is 1 in 5 each time. That's not so unlikely.

: Moreover, I doubt I would've failed to notice him - he would've shot at me
: regardless

But bear in mind that he's inactive for a few seconds to begin with, as seen in my movie. In the meanwhile you could've gone around the corner into the hallway and got involved in some chaos, in which case he could easily go unnoticed.

: Still, I might try some testing at some point soon.

How about now? It doesn't take long if done efficiently. (Note: when doing the spawn triggering, also be doing some checkpoint delaying to make sure you don't then get another.)