This is fairly simple to describe. Browsers are cutting the background colour of the prev & next buttons in half.
I've tried resizing, same effect every time. First image is in MUSE, second is in browser. (Firefox)
Something overlapping the buttons?
Buttons set to be responsive?
If none of these options, we would ask for a .muse with just this issue, one page, no images (maybe only two or use placeholders).
To see what might have happened and tried to reproduce your issue, I had to do different resize options and still I could not reproduce your sight 100%.