PSA: Be super careful with TorqueBoards Direct Drives, almost got very injured again with them

[Minimadness]

Its fairly normal for them to heat up like that on the bench when near full duty cycle because of core losses. See:

Motor Heatsink Development

That is due to core losses at high erpm. Would even happen at no load and high ERPM. Cooling the rotor would have zero effect to prevent core heat buildup.