This reset I noticed so far two quests with little to no monsters/boss, exiting and re-entering something that usually fixes this did not work. So far it's happened on bats and vegapygmy quest in tangled woods. I'll keep an eye out for more.
Also noticed this problem in the Marching dead quest.
Had this problem on the Riser quest the other night, on the first invasion part.
Sand Reefs didn't have the boss spawn for me. :(