I added 50% Less Damage from Humans to dummy.
Flat damage was indeed reduced, while DoT remained its original value.
Compared to previous test in 1.6 PTR, Less Damage from X no longer reduces all duration effects, including DoT, retal DoT, RtA DoT, RD, RR, Fumble, duration of crowd control, etc.
I think Less Damage from X does not reduce DoT is a bug. Not sure whether it is intended that other duration effects are not reduced.