The bug
When you eat a Pufferfish, you will get poison 4 instead of 2, despite dealing exactly the same amount of damage as poison 2.
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Comments 5
Just thought about it some more, and yes, based on the original metric of MC-10755, it is considered a valid report. Can't find it having been reported before either.
While I am aware the Wiki is not a source, it makes a good point: This is likely caused by the half second damage immunity, meaning that the bug is kind of that Poison IV doesn't work correctly at the moment due to that limitation - which may be intentional, but then having anything above II is useless (though according to the wiki, V is faster than III and IV).
https://minecraft.gamepedia.com/Poison