It’s hard to say in words what’s wrong, so here’s a video. This is obviously with “show subcategory list” enabled.
The moment to pay attention to is when I have a category selected, and I re-select that same identical category from the drop-down. You’d expect…
A: Nothing is loaded.
B. The page refreshes.
… instead I get weird option C: It loads the root category index on top of the topic list.