Item entities can glitch inside the piston's head (block 36) upon being moved, if they are inside cobweb/power snow, and under some special conditions.
This issue is specially bad since powder snow being introduced (1.17) but it can be replicated in 1.14 with cobweb.

How to Replicate:

Open the world download attached in 1.14
Power the command blocks, which will summon multiple item entities
Power the piston

Expected behavior:

All entities move towards the next block, and exact same position for all

Observed Behavior:

Some entities glitch inside the piston head. This means they'll all randomly spread across the block.

As mentioned earlier, this issue becomes specially problematic for powder snow (1.17+), since this can be moved by pistons. Powder snow is very commonly used for item aligners, to slow down entities, etc. It is crucial to fix this issue. A video showcasing the issue will be attached too (recorded in 1.21).