This is a regression from 24w36a (where MC-207101 & MC-270183 were fixed).

The area effect cloud spawned by lingering potions no longer appears inside the entity, but above it. Due to this, the player doesn't get any effects unless they jump.

24w36a:

24w37a:

How to reproduce

1. Create a world.
2. Launch any lingering potion above you.
3. Notice how the area effect cloud has spawned above you.