It focuses indeed Mail.app, yet for some reason the Moom keystroke for a tiling action doesn't launch. The keystroke does launch in a normal non Keyboard Maestro keystroke.
Yea, there's something, but my mind and Spotlight searching aren't letting me find it right now. Until I can talk with Peter (to clarify, that's Many Tricks' Peter, not the KM Peter!) who's German, so it's the middle of the night there), the short answer is "don't do that" :).
If you avoid the numeric keypad, you can send keystrokes that Moom will grab. Hopefully I'll have a better answer in the next twelve hours or so.
In modern macOS (since Catalina?), simulated keystrokes do not trigger hot keys for function keys or arrow keys. I'm not sure about number pad keys - I thought they worked, but it is possible they do not work either.
Maybe that is the issue.
In any event, Keyboard Maestro can only simulate the keystroke, what happens after that is up to the system.
I have emailed you privately to discuss some esoteric details related to numeric pad masking that don't make sense to carry out in a forum post. Hopefully we can post a follow-up after we have a chance to discuss what we've found.