Since the advancement window is not moveable to the same extent the achievements window was, the advancement descriptions are able to overflow the Minecraft window which causes the description to be partially unreadable.
This happens with the "A Balanced Diet" achievement if you have the German or French translation turned on (didn't try other translations). It only hides a few pixels, but custom advancements could be affected by this even more.
There are two possible fixes:
b) Change the check for whether the description of the advancement you hover over should be displayed right or left of the advancement to also consider the window size (not recommended)