As noted in the last post in the thread I’ve linked down below, Canon cameras won’t fire the focus assist (internal flash nor external iR) if AI Servo is the selected focus mode.
This makes sense if you understand what AI Servo is and does, but if you don’t know about it and if you get into a low-light situation – as I did this past Wednesday – where you’re depending on the assist, this probably won’t be your go-to settings check.
I killed several minutes double-checking settings to no avail; I gave up and had my shooting partner light the subject’s face with a cell phone.
I didn’t find resolution until I found this tidbit just a couple days ago.