Since we updated wordpress something strange has happened. We have a woocommerce where products have different color options for each part of this, and when you select a color, the image with that color appears above the product photo so you can see how it looks.

This has always worked without problem, but since an update the images in a small (mobile) resolution have fine gray lines, and when choosing several colors it is noticed that in the background there are these lines. The problem is not in the photos, so I tried other photos that worked before and now they don't.

This is before adding colors (no lines):

This is what happens when you choose some colors:

Where can be the problem? Thank you for reading.