When adding a data pack to a new world, the back button on the experimental feature requirements page has no button background for certain window sizes. The dirt background appears to overlap at some point when resizing the window.
I've narrowed it down to being introduced in the first snapshot for 1.20.2, 23w31a.
https://minecraft.wiki/w/Java_Edition_23w31a
In this snapshot there were various changes to buttons, so perhaps as a side effect this cropped up.
This bug appears to be fixed in 24w09a. The back button is now completely shown when the window is resized.