Bought new today and haven't shot it yet.

When hand cycling the gun, the trigger isn't coming far enough forward to let the safety come back out and engage. This is when holding the trigger down, cycling the slide, and letting off to reset the trigger (How I normally shoot).

If I pull the trigger, let go of the trigger, and cycle the slide, the trigger safety will reset.

I swapped parts around with my glock19 to see if I could single out the issue. The mr920 flat shoe and arm will work in the glock 19. The stock glock 19 trigger will work in the mr920. I swapped the connector bars and that made no difference in safety reset.

Potentially the striker spring? Both feel comparably stiff though when pulling them back with my finger. Otherwise maybe the frame binding a bit in the mr920?

Currently have the g19 trigger and arm in the mr920, which honestly feels crisper than the mr920 trigger, would have liked to swap to the flat shoe though.

Any idea why it's not able to reset?

Trigger spring too heavy??  In the OE glock design the trigger spring HELPS pull the trigger, that's why it stays back if pulled and released....if the spring is too heavy, the trigger may not travel sufficiently to reset the trigger safety.
Conversely, you can trim a little off the back edge of the safety so it engages with the reduced trigger return travel...