Why are product variant buttons not updating correctly on mobile?

Hello,

I am using the theme ‘Fashiondable’ and having issues with my product variants.

Some products are showing with drop down variants, while others are showing with a button.

On mobile devices, the variants with a button, whenever someone changes their selection (From say Small to X-Large) the button sticks on the first selection and highlights both. When the person adds it to cart, the smaller, original, highlighted size is whats added to cart.

Does anyone know why this is happening? Or how to rectify this as its causing issues with customers buying the wrong size.

https://www.smashandpass.co.uk/products/scottish-strength-crop-top-olive