I am having the same problem with Merten Connect 505xxx/506xxx battery powered wall buttons/transmitters.
During the initial inclusion I get the Device Interview Complete (most of the times) but then during operation of the device and when I am either trying to set up associations and/or change configuration parameters the Device Interview status suddenly changes to incomplete.
I think that this is why it would be good to save a successfull interview status, especially with problematic devices like these Merten switches.
When the device interview status has changed back from Complete to Incomplete the device does not seem to work properly anymore and I can not get it to activate events in the Trap on Events app.