The bug

Item entities do not collide properly with scaffolding. Even though there should be enough space for them, the items are pushed out.
This can also be seen when trying to throw an item into a scaffolding block.

It appears that projectile entities (e.g. arrows) are not affected.

How to reproduce

1. Place an item in the center of a block

   /execute align xyz run summon item ~2.5 ~0.1 ~0.5 {NoGravity:1b,Item:{id:stone,Count:1b}}
   

2. Place a scaffolding block at its position
   → The item is pushed out