Shulker boxes and other blocks in the end exit portal when it changes state are not dropped as items

The bug

1.21 pre-release 1 finally fixed MC-902 by making the replaced blocks drop as items rather than outright deleted as previously. However, this behaviour does not apply to blocks placed within or around the exit end portal when the dragon is killed or resummoned.

The other kind of end portal is also affected, which is reported under MC-227097.

How to reproduce

1. Enter the End
2. Place your precious shulker boxes inside the end fountain
3. Kill the dragon

Expected results

As the end portal activates, the blocks replaced by the end portal would be dropped as items and would be sent through the portal.

Actual results

Your items are now gone forever. Well done!

Further notes

Also happens when the portal deactivates during the summoning ritual.