I’m guessing that when you said you put the ik right before the output pose you mean your took the local to component section of the head rotation and put it right in front of output pose - so it doesn’t happen within any of the cached poses like this
I also figure your seen this post which affirms this idea: Head Look & IK on montages
If this is what you tried then at the moment I cannot think of that the issue could be. once you’ve calculated the Default pose it should serve as the starting point for any new additions. My guesses are to first look at any settings within the transform node, and also make sure the head rotation is still being passed in during the montage (maybe print it out to make sure the value is still updating in the animBP). but those are just additional thoughts you have likely checked.