First, this is a duplicate of MC-123229 which is a duplicate of MC-10755. However, those issues seem to be really old and I want to precise more things compared to MC-123229 that only describes levitation effects. I'm sorry if this isn't helpful but well...

In a singleplayer world, version 1.17.1, full vanilla. This also happens on 1.16.5.

I use /effect give @s minecraft:resistance 1000 128 true, then I check nbt tags and I see the given amplifier is -128, everything is fine.

ActiveEffects: [{Duration: 19853, ShowIcon: 0b, Ambient: 0b, ShowParticles: 0b, Id: 25b, Amplifier: -128b}]

Then I quit, I join again and I check data tags again, this gives something like the following, you see the Amplifier is back to 0.

ActiveEffects: [{Duration: 19853, ShowIcon: 0b, Ambient: 0b, ShowParticles: 0b, Id: 25b, Amplifier: 0b}]

This error happens on every negative Amplifiers, from -128 to -1 (from 128 to 255 in the command /effect). I tested on levitation, resistance and regeneration effects. It's clearly weird on levitation effect, with amplifier -128 you are sticking to the ground and then when you disconnect and reconnect, you fly with levitation level 1 (amplifier 0).

I hope this can be fixed and not marked as "won't fix" because this behavior can break maps, if someone quits and joins (or crashes) he will have a different effects (e.g. he has high resistance or he has to be stuck to the ground). It seems this happens on join only, I check the player data nbt tags when I was logged off and it was set to negative number as expected (but goes to 0 when I relog then save world again).

Thank you !