I'm having the same issue.
To test i set the speed to 50 and padded one motor by 50 so i should have seen only M2 drive, but it continued to drive as normal veering to the right.
when i slow it down to 50 i notice that M1 starts a short time before M2, in fact i have been seeing instances where M2 will not drive at all when trying to control the two motors individually.
Do i just have a defective part? and if so how do i get a replacement, i am supposed to be doing a product demo for this at a maker faire to sell them and i'm having nothing but problems getting it up and running.
(i'm running the windows version of mBlock by the way)
I was able to correct the drive by reducing the left motor value by 3