Minecraft wiki entry for Effect, associated text: Due to the way this amplifier is stored in NBT, amplifiers of 128 to 255 (levels 129 to 256) in NBT tags (e.g. on potions) are treated as if they were −128 to −1 (levels −127 to 0) respectively.‌[Java Edition only] The /effect command initially applies such amplifiers in their positive-valued sense but this is later converted to the negative-valued interpretation, leading to inconsistent results. In Bedrock Edition, amplifiers above 127 translate to positive levels, like other amplifiers.