the L298 requires the pwm signal to drive the 2 signal pins not the enable pin, or at least that's what I had to do at 100Hz.
If the motor is not under any great load then disconnecting it (PWM to enable) might not look like it is doing anything but it is. Shorting out a motor at PWM speeds is something I would not have considered doing. I don't like the sound of it but can't come up with why at the moment. Maybe it's too late.