The bug

Programmer Art, as of 1.20.5, attempts to recreate the pre-1.20.5 UI as closely as it can. However, the header and footer separators, seen in for example the world creation menu, appear different in the latest 1.20.5 snapshot than they did previously.

Unsure as to whether this is intended or not as I don't know how closely Programmer Art is intended to recreate the old UI.