Due to how the Tab container was implemented, the container expects two or more tabs at all times as part of the default schema. So when you delete the second tab in the Designer, you’re only deleting the visual representation of that tab; it still exists behind-the-scenes, and when the View is closed and then opened the second tab is resurrected.
It’s sort of like what happens when you remove the positional properties of a component - they still exist
and if you add the property back in you’ll see that it retains the value it had when you deleted it.
I’ll speak with Dev to see if that second tab could possibly be removed as a requirement of the schema.