I’ve had exactly the same problems with a Crumble / robot kit. Looking over it with a multimeter, there is no voltage drop either on supply or motor output, when driving the motors at full power.
Tellingly though, with just one motor hooked up, it can run at 100%, or 60% with both. Guessed it must be a regulator or current limiter. Any more info or work-arounds would be great.