I use Spotify on 3 mobile devices - an iPod, and iPhone, and an iPad.  For reference, I never try to use it on more than one device at the same time.

I seem to have fairly consistent issues where my offline music is removed (ie. the dreaded 'you have offline music on more than 3 devices').  The suggestion of the error message is simply NOT the case.  I don't own more than the 3 devices above, and no one else uses my Spotify account or has my details.  I've also checked the 'reset' tool provided by Spotify and it lists only these 3 devices.

I have a suspicion that this has to do with the version of iOS on the device or the model of device itself.  For example, restoring iOS on a device or upgrading to the latest iOS release seems to trigger it.  Also on a nearly annual basis the device itself gets upgraded (iPhone 5 to iPhone 6, for exampe).  

Is there some 'procedure' I should be following to prevent my offline music being deleted every time a change is made to a device or it's operating system?  If not, it seems Spotify needs to change the way devices are registered to prevent this happening.  I take a great deal of time selecting what music I use on each device, and it's very frustrating when I go to use the membership I pay for only to find I can't as the music is missing and I'm not in an area with wifi or mobile access.

Would appreciate any advice from the Spotify team or others who might have experienced the same.

Thanks!