Simplify 3D uses a modified version of GPX to generate the x3g from the gcode. They may have a flavor mismatch. They need to emit M126 if they're using the MakerBot gcode flavor parameter to GPX. The typical slicer emits M106 to turn on the fan which will work for GPX in reprap flavor. If they got this matched up wrong then it is commanding the extruder fan instead (on does nothing because it's on >50C anyway and off is bad and could cause a clog).

Also, the x3g protocol really only has a command for ON and OFF. Any speed > 0 is translated to ON and speed = 0 is translated to OFF.

So, you could see what they're emitting by saving as gcode and looking for M106 or M126 in the gcode file. Figuring out what flags they're sending gpx is harder, but we could figure it out from decompiling an x3g and comparing it to the gcode. It may just be a bug in S3D.