SharePoint App bugs - thinking logged-in users are logged out

Hello -

I'm using a SharePoint site to collaborate and share information/documents with faculty and counselors at a summer youth program. We have a number of issues that appear primarily in the mobile app.

1. The calendar web part does not display. Instead it says "the group calendar is only available to its members. To see the group calendar, join <group name>." This appears for users who ARE members and who are logged in. It is a private group. It even appears for me, one of the owners and the creator of the SharePoint site. Occasionally it will display. This does NOT happen in the browser or on a desktop. The calendar displays fine there. 
2. When this issue occurs, users also can't access some of the content/features (like forms). Sometimes they are asked to log in again if they click on them and it works after they do so, but there is no rhyme or reason to why/when this is happening. User permissions don't matter.
3. Some page links open a blank page. I have gone directly to these pages and copied the link again and edited the menu navigation with that link but it makes no difference. Again, sometimes it will display, but most of the time they are blank. These two pages are audience-specific so it's probably related to the other issue where the app thinks people aren't logged in even though they are. Again, works fine in the browser or desktop

WHAT WE HAVE TRIED:

• Refreshing the pages - no difference other than when it occasionally appears, but then will disappear again next time.
• Logging out and logging back in - no difference
• Clear Account Cache in settings - no difference
• Accessing channel pages directly from SharePoint search rather than menu link - works fine, but any link I put on the main page using the same exact URL loads a blank page. 

People were resistant enough to going paperless and using SharePoint. It's been frustrating to have to troubleshoot app bugs that I have no control over. I spend enough time managing the content, automations, permissions, etc.

Thanks for any help you can give!