Haven't experienced that before. But I do know that with assistive touch enabled, sometimes the screen does take a longer time to dim. For example, if a notification comes in on your lockscreen, with assistive touch enabled, the screen will take slightly longer to dim than if it was not enabled.
I think reason for Apple implementing this is because people who use assistive touch may be users who have difficulty using the phone, and their home/lock button may be faulty. Hence, with assistive touch enabled, the screen tend to stay on longer to provide more time for the owner to react so that he/she will not need to fumble with the faulty buttons to turn on the screen. I highly doubt that this is a bug, but in fact a function to make the iPhone more user-friendly even if the buttons stop working as usual.
Cheers ^^