Eating a singular unstacked pufferfish does not inflict any debuffs on the player. (tested in singleplayer)
https://www.youtube.com/watch?v=I2CRCcnRcQI
Code analysis
Based on 1.11 decompiled using MCP 9.35 rc1
The problem is that the stack size is decreased (net.minecraft.item.ItemFood.onItemUseFinish(ItemStack, World, EntityLivingBase)
) before other methods are called, in particular net.minecraft.item.ItemFishFood.FishType.byItemStack(ItemStack)
which calls the method net.minecraft.item.ItemStack.getItem()
which returns air because the count is 0.
This is a general problem and affects multiple items. Luckily they don't call methods which require a valid item at the moment.
Related issues
Comments

Confirmed