Lightroom Mobile Masking not working

Thank you for the additional notes.  I cannot reproduce the problem on a similar device and same iOS version.  We'll continue to investigate.  The very next thing I recommend is that you do a full iTunes/iCloud back up of your device so that you can always restore your device exactly to the same state as it is today.

Does a device restart help?  Are you open to updating to the latest iOS 15 to see if that changes anything?

Regarding reinstalling the app, I suggested this because you seem to be a paid user.  If you are a paid user (which your UI did indicate), your images, edits and presets should sync to Lightroom cloud storage.  You can confirm your files are synced by tapping on the cloud button and checking sync status.  If your files are indeed synced you can re-install the app and the files and presets should sync back down.  Only free users cannot enjoy this cloud sync feature.  To be sure all your files are synced to cloud storage, you can sign into your same account in a web browser at lightroom.adobe.com, and check that your most recent edits and imports do exist in the cloud.  

If the device restart or simple iOS update do not help, I am curious if you can do a iTunes/iCloud backup of your device and then try re-installing the app.