On iOS, when you link to the official Moodle app or Learn Moodle via the URL scheme moodlemobile:// or learnmoodle:// from another app it states the app isn't installed.

Also if you google search the URL scheme, the installation of the app isn't detected.

But on Android it works fine.

I think it has something to do with iOS requiring Universal Links and not  URL schemes. 

I have attached 2 screenshots which illustrate the issue.