Unset "Default Topic List" setting, or reset to default?

I was experimenting with the category settings and I noticed that “Default Topic List” was not set for any category. I set it to latest and now I cannot unset it to the default anymore. See screenshots below.

Screenshot from 2021-02-23 06-19-22

Screenshot from 2021-02-23 06-19-05

Now of course it’s not a huge deal. But I do like to keep settings to their defaults if I don’t use them, or at least keep them consistent with the other settings. All other categories have this still set to an empty value.

So how do I reset this setting back to its original ‘null’ value? Or might this be a bug?

I wouldn’t worry about the aesthetics of it; the default is “Latest”, certainly not “Top”.