Summary:
In building Mode, if you delete a door from a wall, the wall become hollow.
Steps to reproduce:
Expected Results:
Fonctionnal deleting mode.
Version Number and Mods in use:
0.20 original
System Information:
Welcome to the forum, @Kobalt 
What you mentions is not a feature, so it’s not actually broken. We can’t make holes like that on walls, but by building with slabs you can leave open any holes you want.
We’re aware of the restrictions of the current building editor and are working to have a better one, with improved UX and that allows the player to build whatever they want. It will take a while but it will be worth it.
I will move this to suggestions. Thanks for reporting.
The way I understand this bug report/suggestion is that he removed the door and the hole for the door didn’t got filled in. This happend to me a few times too, but i didn’t reported it because its mostly a update bug that dosn’t always happen. I just placed an other door or window and the hole got fixed and it only happens in late game when the game starts to laag/slow down.
I gues it just takes to long to update and so the remove task gets dropped for an other more important task or something like that. The nice thing is you can build the building with the hole in it by not updating the blueprint (placing an other door or window). 
Interesting, I will investigate. Thanks!
Thank you Prepe for the clarification !