The bug
I have a data pack in my world that contains a lot of advancement trees, and some of them are under the GUI. I just realized that when I hover over the tab to see its name, half of the text is overlapped by the advancement background.
How to reproduce
Create a data pack with lots of advancement trees in a world
/advancement grant @s everything
Open the advancement menu
Hover over a tab that is under the GUI
Look closely at the text
→ ❌ Half of the text is overlapped by the advancement background
Confirmed for 20w14a.