Items always pass through walls (and even full blocks) when pushed for example by pistons.
Watch the video to understand clearly the problem here.
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@unknown You're right. It depends on the direction of the piston.

I'm pretty sure this
Works as Intended
If it doesn't, and they patch this, it will break a whole lot of stuff, because some buildings are hardwired to take advantage of this.

@unknown No. It is a bug. It happens with all items and even if I place a shulker box next to the piston, the piston will break the shulker box and the shulker box will pass through the wall and fall. This is 100% an issue. It doesn't happen in Java/consoles and from a simple understandable point of view, it's easy to understand that this is a problem. Why items should pass through walls? and which buildings are harwired to take advantage of this behaviour?
Thank you for your report!
We're going to be tracking this issue at MCPE-46746, so we will resolve and link this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
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This does seem to vary depending on the direction the piston arm is facing.