With no noticeable pattern, sometimes the Cmd-Tab hotkey for the Keyboard Maestro triggers the OS X application switcher, instead of the Keyboard Maestro’s switcher. Should I actively disable the OS X app switcher hotkey, as a matter of course?
It looks like there isn’t an easy way to turn off macOS’s app switcher keyboard shortcut. 
There is no way to turn off the system Command-Tab switcher. Also, it is not a real hot key (otherwise both would fire), and so Keyboard Maestro intercepts the key specifically with an Event Tap, which is probably how the system switcher does it as well. Because Keyboard Maestro registers its Event Tap after the system, it runs before the system, and thus can capture the key before the system switcher code sees it.
Occasionally you can get into a state where the system switcher registers after Keyboard Maestro (either because Keyboard Maestro registers very early or perhaps because the system re-registers for some reason) which would cause the system switcher to take precedence. Relaunching the Keyboard Maestro Engine should resolve that.
Unfortunately since the system does not offer a way to turn off the system switcher or adjust the keystroke, there is not much I can do to make it more robust.