DRV89xx-Q1 (DRV8904-Q1,DRV8906-Q1,DRV8908-Q1, DRV8910-Q1, DRV8912-Q1) device has internal PWM generator. When PWM Duty is set as FFh(max), it is very close to 100% but there is a small gap.
For example. With100Hz: PWM=FFh : ON DUTY = 9.98ms/10ms = 99.8%
Workaround is to switch to "Continuous mode" then Duty will be 100% ON
Details are attached.