Hello @R111 and thanks for posting to the forum. I couple responses to your post.
While it might not be the cleanest user experience and creates a big of confusion, it isn’t a bug.
… but we are though. Default container is nothing more than the section in which tasks initially get assigned to prior to a user or Rule moving them.
I’ll follow this explanation with a screen recording to make it clearer. The “untitled section” isn’t shown in the List View by default because the layout allows for such. It still very much exists though. Evidence of such is when switching to the Board View, you see the “untitled section” in the far left. In the Board View you would have a floating column with no assigned name to it if it didn’t show “untitled section”. This would likely cause even more confusion.
So when does “untitled section” show? Well… NOT when you just add a new Column in the Board View or Section in the List View. It doesn’t show until you have ANOTHER column in the far left position or move a Section above the invisible section in the List View. You can indeed get it back to the way it original was (where the “untitled section” is no longer listed in List View. All you need to do is move a task ABOVE the first Section in the list view.
Whatever is the top section of your List View or the far left Column in your Board View is the default container for the project. Matter of fact, while the section is invisible in the Task Details you can see where you can associate a task to the “untitled section”.
Your suggestion is essentially how it works but instead of the default container having a specific name it is essentially invisible when it has yet to be named. While it might not make sense at first, I think the way it works today is a very simple and elegant way of doing it… for the most part.
Here is a screen recording as a demo (apologies for being long-winded. I didn’t anticipate it would take 4min to demonstrate 
: Loom | Free Screen & Video Recording Software | Loom