hi,
I’m having this issue while playing animation.
Isn’t the eyeball fixed to the bone of the head?
Or is the variation applied to the character? 
Hi it could also be that the eyes are not a perfect sphere?
You’re providing very little information about your issue
If you want to get helpful advice, please consider providing some context
How is the character rigged, animated, etc.
What exactly is the issue? A deformation? The eyes jumping out? A lighting issue?