Place blocks in the air, right-click to sit, sit away from the bottom, appear under half a block, will fall to death, cannot stand on the block

Which version of Sit? Which Mod Loader? Which Minecraft version?

All versions are the same

The original version, ship, bed, mining car, and descent position are all optimized, but the descent position of this mod is not optimized

I am asking you which version of Sit you are using, as well as which Minecraft version you are using. I'd also like to know if you are using Forge, NeoForge, Fabric, or Quilt.

Stand in the air when leaving, not on blocks

1.20.6fabric-sit-1.20.5-26

"Sit" always puts you back where you clicked the slab from. This is in order to prevent sitting being abused. It's supposed to be purely cosmetic, and not give any gameplay advantage. As far as I can tell, the behavior you see is intentional.