Notebook list on mobile app (iOS) does not match notebook list on desktop

I have a couple of notebooks on the Windows desktop app that are not listed in the iOS app. The notes themselves seem to be in the iOS app.

So for example if the notebook name is NB_Test, the notes from NB_Test are in iOS app, and if I click on one of the notes, NB_Test is shown at the very top of the note. And if I click on the dropdown at the top of the note to show the notebooks I can move the note to, NB_Test is in that list.

If I check iOS app's Sync Status, NB_Test is listed under Folders.

So why wouldn't NB_Test show in the main list of notebooks? The behavior makes me think there's a flag on notebooks that determines whether they show in the mobile app's notebook list. Another notebook that doesn't show in mobile app is 'Welcome! (Desktop)', which is the notebook created when you initially install Joplin desktop, though of course all the notes in that notebook are in mobile app.

Joplin iOS version 12.12.3
Joplin Windows version 2.12.19

Update: I tried renaming NB_Test to NB-Test, and now it shows up in the notebook list in the iOS app. Funny thing is now the 'Welcome! (Desktop)' notebook also shows up in the notebook list in the iOS app without any modification of that name. Not sure what's going on, but all's well that ends well.