It's a bug. If an extension other than your preferred extension was previously selected, the "list of 1 thing" that is supposed to show your preferred extension instead shows an empty slot.
There's no workaround AFAIK, but (as you found out) if you use something that shows all extensions, such as Control Center, then you can choose your extension. The next time you try it from your app that sets the same preferred extension, it will show up.
Rather than trying to explain that to users, it's probably better to not use the preferred extension setting at all, until the bug is fixed. You can tell your users to choose your extension from the list, which is a little easier to explain.
Though I would reproduce this on iOS 12.1.x, I can't reproduce it on iOS 12.3.0, so I believe it has been resolved. Can anyone else confirm?