I have a 2560x1440 pixel display. When Minecraft is in fullscreen, every configuration menu button looks like 2 separate buttons. Shrinking to a width of c. 2500 pixels connects the "buttons".

This happens for me as well, but only on GUI Scale 1 or GUI Scale 2.

When playing on GUI Scale 3+, it looks fine. It's still a bug, but that's a temporary fix :^)