Effect amplifier is read/written as byte from NBT despite being stored as an integer

The bug

Effect amplifiers are read as byte (-128 to 127) instead of as an integer (-2,147,483,648 to 2,147,483,647) from NBT.

How to reproduce

1. Give yourself jump boost with an amplifier higher than 127 using /effect

   /effect give @s jump_boost 100 128
   

2. Try to jump
   → You cannot jump (see also MC-90198)
3. Reopen the world
   → You now have jump boost with an amplifier of -128 when read (though will be considered 0 when an amplifier obtained via NBT is less than 0)

Code analysis

Based on 1.12 decompiled using MCP 9.40 PRE 1

The methods net.minecraft.potion.PotionEffect.readCustomPotionEffectFromNBT(NBTTagCompound) and net.minecraft.potion.PotionEffect.writeCustomPotionEffectToNBT(NBTTagCompound) reads/writes the amplifier as a byte, while it is stored in-memory as an integer.