Downloaded files are not appearing in the Downloads folder in Files app

On my iphone, downloaded data files or images do not appear in Download folder in Files App.

• I am using the Post Standard newspaper app. Vendor is clueless and has no technical support.
• In settings, Safari is set to download to iPhone.
• I have an empty download folder inside the File app.
• When I select download icon in app I can see the progress increment to 100% but they don't appear in the folder. In the app the download icon is then greyed out indicating download complete.
• Can't find the files anywhere.

[Re-Titled by Moderator]