Hi! I am experiencing exactly the same issue as related in https://community.spotify.com/t5/Ongoing-Issues/Multiple-registrations-of-same-device-in-offline-dev... that was marked as "not an issue" and closed.
In my case, the fix provided in https://support.spotify.com/uk/article/listen-offline-troubleshooting/ wont also apply.
It is more than unconvenient because my music dissapears once and again by surprise even though the limit of three devices is not truly reached.
This issue has been reported more than once by many different users ... is it already a fix for it?
Thanks and regards!
Laptop HP i7