Is this really an exploit though? I mean in reverse there are times when players are doing an NM but some end up pulling a few enemies not part of it, so they may unsync to get rid of them and then focus back to the NM again. You could do the supposed exploit with any FATEs actually, so if you wanted to kill specific enemies and be rewarded, you could lead them to a FATE and sync down to not suffer any penalty. Granted the FATE situation isn't the same as light grinding, but the method has always been in the game it's just been adapted to Eureka.