Animation not playing specific bones

I have a mesh without any animation data that has the same rig as a few animations refusing to animate specific bones.

It is an ostrich, with a clean mesh without any animation data, as an FBX. It is rigged and working with the avatar from the FBX that has a "walk" animation. The mesh has its neck and head straight up, while the "walk" animation has the head stretched out in front of its body, the difference being the neck as a vertical line (idle) and a horizontal line (walk).

When playing the "walk" animation, the neck is refusing to animate as it is in the animation data - with a horizontal outstretched neck.

If I use the mesh that has the "walk" animation tied to it (FBX), then the horizontal outstretched neck works, but the idle animation is then screwed up.

Setting "write defaults" has no effect whatsoever.

Does anyone know how to fix this?

Using Unity 2019.4.6f1