With OpenNMS 15, no outages records are getting written to the outage table of the database.
For example, on demo there are a number of devices who have an outage due to software needing to be updated. RTC correctly shows the outage, but the outages list does not and the availability of those services shows 100%.
This is a pretty nasty bug as it affects all aspects of availability calculations. The lost service events, however, are still being generated. Not sure if this is affecting node down or interface down events.