Both of these ideas might be worth a shot.
First idea
The motor brake type of hold is a bit of an odd thing.
How does braketype::hold work? and thoughts about it - #2 by theol0403
The active usage of the motors could cause the motors to overheat (and lock I suppose). I would suggest that you change the motors to use braketype::brake instead.
Second idea
Our motors have locked up from ESD (electrostatic discharge). It happened suddenly for a good couple days. The brain port would be damaged, we switch the brain port the motor si plugged into, then the next brain port gets damaged. We resolved the problem by putting electrical tape around the port on the motor and switching the port. Haven’t had a problem since.