File links generated in iOS app are not working for an external user with permissions

Hi--

A bit of a weird situation I cannot figure out. We have an external user who has access/permissions to files and folders. When we send them a link generated in the iOS SharePoint app (copy link or Share) , they get an error saying they do not have permissions to view the file.  HOWEVER, if we generate a link for the same file using desktop or web SharePoint, they are able to access the file just fine. 

Any ideas?