Animations in DOS2 have different priorities, and animation overrides are at, or at least near, the lowest priority. So if a character has the DefaultCharacter behaviour script assigned, then in its idle state it will occasionally play the idle1/idle2/idle3 animations, which would probably cancel the animation override. Not sure why it would have worked before though.
On the other hand, a PlayAnimation call from Osiris is pretty much the highest priority you can have for animations, so those are very unlikely to be cancelled.
I also forgot to mention that PlayAnimation also works with looping animations. It simply will play the looping part only once.
