Core declaimed itself

One of my Cores has declaimed itself, twice now. It no longer appears on the Build site ( in the list of devices ) or in either of the Spark or Particle apps for Android.

It does, however, still send events which I can see on the Dashboard. I was able to add it again using the device ID it sends as part of its message.

Any thoughts on what may be causing this ? I believe I had to factory reset it last time, and reclaim it from there.