For those above that reported issues using Android and having to constantly log back in, make sure that the App Permissions (Location, Contacts, Storage) for the SmartThings App are Enabled. Another user in another thread was having issues with constantly having to log back in on Android. I could reproduce this on Marshmallow and Nougat by disabling my app permissions on both OS. I don’t know if it’s one of the permissions or a combination, but when all are enabled he no longer has to manually log back in.
Hope this helps one or more of you.