The bug

When you get a potion as junk from fishing, it is "uncraftable" because it has no Potion tag.

How to reproduce

Summon for example a mob with the junk loot table and kill it until you get the potion.

/summon villager ~ ~ ~ {Health:0.1f,DeathLootTable:"gameplay/fishing/junk"}

The reason

The loot table minecraft:gameplay/fishing/junk.json is not setting the NBT for the potion.