Shulkers dropping there items when broke with a piston.

https://drive.google.com/file/d/1lVhNppjDwSGPDg0W2jiSRBfj7NYCk0sp/view?usp=sharing

Ignore the audio. 

I was trying to build a randomizer using shulker boxes but had the issue of the items coming out of the box. I couldn't find any fixes online and looked it up and they should not do that. I don't know to do and I need a randomizer for my build.