Spotify uses the Episode URL rather than the GUID as its way of identifying episodes and is noncompliant with IAB guidelines. Per the Podcast Player recommendations in the IAB guidelines, the recommended method is to use GUID: Use the GUID—as opposed to episode URL, title, publication date, etc.—to identify new episodes in the RSS feed that should be automatically downloaded to a listener's device. The GUID is designed to be persistent regardless of changes to hosting environments, titles, or other details.
In layman's terms, what this means is any time your podcast episode URL changes (changing hosts, adding/removing a prefix) Spotify will re-downloaded episodes when episodes are in a listener's downloads (sometimes these episodes are referred to as "available offline" episodes) and when users resume a podcast that they had previously started but not finished.
This issue impacts all hosting providers and podcast publishers should anticipate a spike in downloads from Spotify variable in size, depending on the number of listeners that have your episodes downloaded, when migrating a show and/or changing or adding a 3rd party analytics prefix URL.