The bug

The XP orb spawned from fishing is spawned (0.0, 0.5, 0.5) relative to the player, rather than the (0.0, 0.5, 0.0) probably intended.

Code analysis

This is caused by adding 0.5 to the Z on the line that spawns the XP orb in net.minecraft.world.entity.projectile.FishingHook.retrieve.
^{Keyword for search: experience}