I've had this happen twice now so would really like to avoid it happening any more if I can help it.
This should be a normal gradient, but as soon as the head moves from it's starting pose, it glitches up.
Before moving:
After moving:
I'm using Anime Studio 8 Pro and I know the most obvious thing to try is to update version but just wanted to know if there was anything else I might have missed.
Gradients work as images, they bend and move with bone strength.
Add some bone strength to the head bone. If it already has, then the problem is the gradient is being deformed by many bones strength.
On AS 8, you can bind the entire eye layer to the head bone. That way the eye and its gradient won't be deformed by any other bone.
In AS 10, a simple solution would be to select the layer with the gradient, then select the bone of the head and go to the Bpne menu > Use selected bones for flexi-binding. That way the software would use only that single bone to move the gradient.