Single page variation selected but not active

Hello,

We are having an issue mainly on mobile devices.
The problem : When going in to a product page (single product page) and there are multiple variations when clicking on a size variation it becomes clicked but the “add to cart” button is not active.
If I go ahead and select another variation or go back to my original selection then the “add to cart” button becomes active.

I’ve disabled other plugins, plus caching plugin and the problem is still there.
Attached I am sending an image of the problem.