According to Apple, the iPhone 11 (I have the 11 Pro Max) records in stereo. I noticed that the clips I recorded with the iOS camera app are stereo. However, clips recorded with the Movi app (or Filmic Pro) are mono. Even if this might not make a big difference, I'd be really curious to know why this is. My search here on the form didn't turn up anything.
Not sure about this, as this applies to the Movi iOS app as well. I did send the same question to Filmic support, but didn't get an answer yet.
Just a wild guess then, neither app has been updated to record to the iphones potential. Filmic should update eventually, I really wouldn’t count on the Movi app to support anything.
I received an answer from Filmic support in the meantime. The camera API (that is used by Movi and Filmic), does not yet support stereo recording. They are hopeful Apple will release this functionality for third-party apps in the future. Just to clarify: external microphones do not have that limitation.