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

The bug

Effect amplifiers are read/written as byte (-128 to 127) from/to NBT instead of an integer (-2,147,483,648 to 2,147,483,647) which is how it is stored in memory. This causes a change is behavior when the effect is reloaded from NBT is the amplifier is above 127.

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.