When a player has a positive amplifier of levitation (In my case I did /effect give @s levitation 10 3 to achieve an amplifier of 3b), and it gets replaced with a negative amplifier (I did /effect give @s levitation 15 252 to get an amplifier of -4b), the player still levitates up with the same speed for the left duration of the positive levitation effect before starting to float downwards from the negative levitation.

To replicate:
1. Apply a levitation effect with a positive amplifier with the command I typed above (can have different duration)

2. Apply a levitation effect with a negative amplifier after that with the command I typed above (can have different duration)

3. You'll still float up despite having the negative amplifier for levitation, and will only start going down once effect given by the first command ends