App Updated date

Probably because Garmin autoupdated it as it is defacto compatible with recently announced new watches such as the Descent MK2, witch is on your compatibility list.

The little gear indicates your app was auto migrated to a device, and the date indicates when that occurred.  The MK2 was just announced, so it could have been for that (check compatible devices for your app).  For your app, there is a setting if you don't want auto migration to occur:

I guess that explains why my ERA listing suddenly disappeared.