Hey Russ
Almost exactly one year later, I'm experiancing the same problem. I have a map application with a host feature service that is editable for my AGOL group.
I was able to view and edit it with the Collector App, but when by mistake I removed the editing capabilities and later readded them to the hosted feature layer, the map disapeared permenantly from the app. Any ideas for solutions?
EDIT: It seems there ws n issue with concerning the rights on the hosted feature layers. Some layers were open to everybody, others were only for a certain group. I've recreted the app and now it works!