Villager's XP bar for trading will sometimes go past the GUI when you, I think, keep trading once they level up, but before the GUI updates with the level up.
Video by violine1101 demonstrating the issue: https://youtu.be/Lk7wV0AD19I
Villager experience bar out of gui
Villager experience bar displays incorrectly
Villager trade GUI is bugged
Visual bug with raid boss bar
While trading with villagers, the experience progress bar shoots beyond the gui