owntracks (tested on ios version 9.6.3/de_DE) publishes single waypoints under the topic owntracks/<user>/<device>/waypoint (singular).
owntrack publishes a waypoint export (publish) under the topic owntracks/<user>/<device>/waypoints (plural).
the owntracks component did not catch my waypoint export to mqtt, only single waypoint updates (i.e. after editing a waypoint or creating a new one). these single waypoints were rejected „because of missing or malformatted data“. when i changed the WAYPOINT_TOPIC to 'owntracks/{}/{}/waypoints', owntracks imported my published waypoint list, after i triggered it under Setting / Publish Waypoints.