It does on some bosses. The ones without the "reward chest". Items are affected by double loot, like: Warzone 1-2-3 bosses, Killing in the name task bosses
Bosses with reward bags don't work like: wz 4-5-6, grave danger bosses, vengoth bosses, ferumbras etc..
If you have boss points I recommend to do the boss "the many" give a good loot even if no rares at all and is quite near to ask for the task again (don't sell the egg of the many now the price drop a lot near of double loot event).