The bug

I noticed that after reloading the world, my resistance 255 effect wasn't working, in fact, after i checked, its amplifier reset to 0.

Steps to reproduce

1. Give yourself an effect which amplifier is greater than 127
2. Reload the world
3. Do /data get entity @s ActiveEffects
4. Notice how the effect amplifier is 0

Possible cause

This might have to do something with how signed bytes work.
Because we're entering unsigned bytes (0->255) in the /effect command as the amplifier, but the amplifier tag itself is a signed byte (-128->127), the game writes a negative value to the amplifier.
So, 128 is -128, 200 is -56 and 255 is -1.
However, when the game checks the amplifier, and sees the negative amplifier, it treats it as 0.