I've spent a lot of time on this and think I've discovered a workaround. The problem seems to arise when you have a mailbox connected via desktop Outlook and iOS Mail. If you let desktop Outlook specify an Archive folder it seems to set a MAPI property on it that iOS doesn't like and it drops the archive folder from the list of available folders. When I say archive folder, I'm referring to the feature of modern email clients where you can either "archive" a message or "delete" it, not the archiving feature of Grommunio.
My hack workaround to fix this was to create a new "OutlookArchive" folder in my account. Then use MFCMAPI editor to change the designated archive folder assigned to my account to be the new "OutlookArchive" folder. Once I did this my previous Archive folder reappeared on iOS Mail. Since I wanted both email clients to send archived mail to the same Archive folder I removed the standard "Archive" button from Outlook and instead created an Outlook quick step to move the selected message to the "Archive" folder. So I use that quick step to archive emails instead of the normal Archive button.
I'm glossing over a lot of the details, especially the MFCMAPI part to keep this post simple. If someone down the road wants more detail just post here and I'll provide it.
Why all this is happening is still unknown, I've only ever had this problem with Grommunio, so I'm guessing there's some issue there or a lack of support for archive folders in general within the Grommunio platform - I'm not sure. I spent hours going through the PHP scripts that run the grommunio-sync feature but didn't come up with any solutions, but that's mostly due to my unfamiliarity with the grommunio backend I imagine.
For now my mess of a solution will suffice, but I'm certainly open to this issue being properly fixed.
