The only DMM that have a derating is for the Dyn2 which are low voltage drives and use the same motors up to 750w
That was my thoughts as well but you said it did it with no pulley so I did not post, servos in general don't react well if the inertia is around or close to the motors max
You did not say in your other post if you are using a Braking Resistor which would help when slowing down, if not it will fault out
DMM have done extensive inertia testing so will know what will work and what will not, I built most of there test rig for there inertia testing