The problem applies when users try to open, for example, a Word document from a SharePoint library (on-prem 2016) where an Office Online server also exists on the farm.
The first time a user tries to open a document the user only see a green loading circle that goes on forever without displaying the content of the document. But if the user go back and try to open the document again the document opens immediately. The problem applies to all libraries but only the first time the user tries to open a document (my immediate thought was caching).
When user opens a document in the browser on mobile device the url changes from the SharePoint site url to Office online url.
Tested with Edge browser on mobile. No problems opening documents from PC.
Also tried to choose that all documents should opened in the desktop version but this does not seem to apply to mobile devices, and the documents are always opened in the browser (also downloaded Microsoft Office for iOS).