What version of grbl are you using? If it has the $30 grbl setting, it may be set to 1 which is what it sounds like is happening. This will send the laser to full power with any S value over 0.
I recommend you use Larry’s latest Spindle/Laser grbl 1.0c that can be downloaded from here:
https://discuss.inventables.com/t/xcarve-jtech-grbl-update-9-1-2017/28943