The outline of the button is just marginally taller (I think 1 pixel on my 1080p screen) leaving a transparent gap that shows the background underneith. The screenshot highlights a place where it is very visible but this issue happens also on the bottom of the button. This is only for java edition and does not affect any of the other buttons.

This issue was also pointed out in report MCL-17953 but was ignored even though it is clearly visible on the first picture.

Recreation: just open the launcher and look at the play button. If too hard to see, get closer, squint your eyes or use glasses.

Mojang pls fix