Sometimes bombs don’t push bosses, It doesn’t matter which, incendiary or black powder.
This bug does not depend on whether the person throwing it is a host or not.
EDIT:
It seems that this bug affects not only the grenades, but also the shouts of Kruber and Saltz. 2 shouts and 2 grenades - Spawn just don’t care.
Experienced this several times last night. Chaos spawn is eating someone: I got a bomb and throw it, spawn just shrugs it off.
Same here. This is an existing issue, I experienced it too. Happens not only with bombs, but anything that supposed to stagger. For example got rekt by 2 monks because animosity didn’t pushed them.