The bug
When the obsidian platform regenerates and sets a cuboid above it to air, all of the blocks that occupied that space are removed as to create a safe air space for the player. However, none of those blocks actually end up being dropped as items, meaning that any blocks placed inside of this "danger zone" will be lost permanently if another player or entity enters the dimension.
How to reproduce
Create a mechanism to continuously send entities through an end portal
Enter the End
Place a block on top of the obsidian platform
Wait for the next entity to enter the End
Expected results
The block placed on the platform would be converted into an item entity.
Actual results
The block is no more. It has ceased to be.
Linked issues
Comments
I can confirm this behavior.

This is a feature request.
Very closely related to MC-160140.