Imported Rig Issues- Head Bobbing in Blender turns into glitched size changes

I’m having an issue with a rig I imported from Blender. The character’s head is not connected to the by by a neck, allowing it to move freely. This works perfectly fine in blender, as seen in this video: https://youtu.be/o6wj3uYeYjA

But when the same animation is exported to Unreal Engine 5, any up and down motion instead results in the head exploding outward in scale, with the eyes not following the motion. See here: https://youtu.be/0pgPuZxyrYo

This is the only bone of the rig that has this issue, and only with this movement. I would just really like some help finding the cause, and hopefully a fix.