Make sure you are not opening a duplicate.

• I have searched the issue tracker and did not find an issue describing my bug.

Description of your suggestion.

Fill the dungeonz dimension with bedrock or some other block (maybe have a config for it if you want).
This would make it so dungeon creators dont have to wrap each of their structures in bedrock or other unbreakable blocks to prevent their players from accidentally falling into the void.
This isnt as much of an issue if you want your dungeon to be unbreakable in general, but id prefer to give my players the freedom to pull apart my dungeons as they would with any other overworld dungeon.
Also might just be me, but knowing that there is just an empty void behind the walls, rather than something like endless bedrock or stone, breaks the immersion a bit.

Out of performance reasons the world will stay empty. When you build your own dungeon you can do whatever you want ;)