The bug

If you start fishing in a jungle biome, you will be able to catch jungle loot on that cast, regardless of the actual biome of the water source. Even if the player or bobber moves significantly after casting. And likewise, you will fail to catch jungle loot, even while fishing in a jungle biome, if you start fishing outside of one.

How to reproduce

1. Download and open the attached world: JUNGLE BUG.mcworld
2. Stand near the first beacon and begin fishing in the river.
   →  You will catch bamboo (I modified the jungle loot table to always provide bamboo).
3. Stand near the other beacon across the river and start fishing in the exact same spot.
   →  You will catch regular fishing loot

Cause

This is caused by an extremely naive check for jungle biome in the fishing bobber's entity_spawned event, which permanently alters the hardcoded loot table it draws from. A possible fix is to expose biome access in the loot table itself (i.e. what Java Edition does).