This is not a bug. Scripts of zoom effect should take some time to replace featured product images to new one of the variation that you selected as default. If your default variation has the same image as featured product effect – http://prntscr.com/io49y0 you do not see this situation, but if these images are different – http://prntscr.com/io4aoy you will see “effect of image changing”. This is independent of a category, and this can’t be changed.
Regards