I deleted the group in the app on the phone, created a new group. Insert the new group ID in the server settings (interestingly, the ID disappears immediately after I finish typing the whole code, but its there if I leave the setting screen and come back).
But I still get the same behavior. Notification on my phone (which is great, even has the message info in the notification), but nothing listed in the app. The app looks as if it has not received any notifications.
BTW, I have a Google Pixel with the newest Android 7.1. Might that be making the app not display the messages correctly? I should try on an older phone.