It took us several days to get to the bottom of what has happened:
Apparently, Anchor uses automated detection of content;
Apparently, us publishing the music track on Spotify made the track recognisable as “original music that has an owner”;
Apparently, Anchor (owned by Spotify) has no way of connecting the dots between us owning the podcast and us owning the music;
Apparently, Anchor’s algorithm is to immediately nuke the account which is found to contain music (without bothering to explain the reasons), and let the support deal with the possibility that a mistake has been made by the auto-detection routine.
In the end, we managed to get some real humans at Anchor to listen to the content that they have removed, and the account has been restored.
My takeaway? Music seems to be a very special kind of content that carries a lot of risk through association. Also, if you use automatic detection, maybe try to “quarantine” the suspected content and email the owners, instead of just killing the whole thing without any explanation.