The thing to remember when looking at motor torque-speed curves, is that the axis showing speed is not the speed you are commanding the motor to run at, it’s the speed the motor is running at due to increased load. Here’s are the original theoretical curves we calculated for the old 393 motors many years ago. (which is drawn differently from the V5 curve above).
It gets more complicated with V5 motors, generally you are asking the motor to run at a requested speed and it is using closed loop control to try and maintain that speed, internally the motor will increase current (and therefore torque) as necessary until we hit the current limit (which may be reduced as the motor heats up). When using raw voltage control, it will behave more like the old 393 motor except we still impose current limits, that’s why the blue and green lines on the V5 graph become horizontal.