Summary:
Throwing/shooting a projectile while having a downwards/stationary levitation effect makes the projectile shoot straight up at high speeds.
Steps to Reproduce:
1. /effect @s give levitation 1000000 (value)
2. get a bow and arrow, splash potion, or similar
3. use the bow/potion/similar
4. Make sure you are looking up to observe the entity flying away from you.
Observed Results:
When shooting a projectile with the downwards/stationary levitation effects, the projectile shoots completely straight up.
Expected Results:
The projectile should be moving in the direction that the player is looking.
Notes:
MC-10755 does not duplicate this issue; the levitation effect itself works as intended despite being at high levels: the player moves downwards or hovers in the middle of the air. Instead, what is being reported is the projectiles use during the intended effect.
Related issues
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Comments


This is a duplicate of MC-10755 as there is no way to get the stationary levitation effect without going higher than effect level 5 which is unsupported

As you can read in my Notes, this issue is more about the projectile than the effect. I understand that one needs to apply the levitation effect at higher values, but the levitation itself works as intended.

There have already been other reports about this (e.g. MC-98231, MC-95340 and MC-95370) and we have resolved all of them as duplicate of MC-10755.
MC-10755 is in general about high effect amplifiers being unsupported. They may work correctly, at least to some degree (as it is the case here), but any issues caused by them won't be fixed by the developers.
If you can reproduce this with an effect amplifier which can be obtained in Survival (without commands or datapacks), feel free to let us know or create a new report.