Hmm I had written a reply but for some reason it doesn’t show up anymore. The core point was, that if your assumption is right, and the head bone transform interfers with the camera rotation, than that’s a bug.
The expected behavior ( if “Use Pawn Control Rotation” is checked on the Camera Component), the head bone rotation shouldn’t interfer with the camera rotation, regardless to what the bone rotation is set to. I mean, that’s the point of that setting, isn’t it? And this setup was working just like expected from 4.8 till 4.14.