What I expected to happen was...:
I would come back from the nether and the items would be in my shulker box.

What actually happened was...:
Shulker box items disappeared when the box was left in the nether and I returned from the overworld.

Steps to Reproduce:
1. Leave shulker box - I had two dyed ones - full of items in a dimension (works for any, from nether to overworld, overworld to end, etc.)
2. Travel through portal, stay for a few seconds
3. Go back through, the box should be empty of items