[Kevin Sevcik]

The 9012 are the ones with thermal protection. It could very well be that the motor has a somewhat different opinion of what's too hot. Remember that the motor can be a lot warmer on the inside than casing would seem to indicate. All the heat is generated in the coils, that aren't actually touching the casing much at all. Particularly, the fact that it shuts off quicker the longer you run it indicates that it's probably the thermal protection.

If you really want to check, run the motors till they shut off, then blast them
with cooling spray till they're cold to the touch. Run them again and see if they run longer.