How to reproduce?

• Give yourself any effect with a level >= 128: /effect give @s jump_boost 999999 128 true
• Leave and rejoin the world.
• Now the potion effect acts as if it were level 1 when it is supposed to act differently at level >= 128.

For example the jump_boost effect at level 128 prevents the player from jumping. But if the player leaves and joins the world, they can now jump as if they had the effect at level 1.
If you put a level 128+ resistance effect on the player it is impossible for them to take any damage. But if they leave and join the world they will now be able to take damage as if they had the resistance effect at level 1.

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