Thanks for the confirmation.
I've been doing a bit of reading on this.
None if the files are are password protected, but the tracks have been tagged with mp3tag to try to more professionally produce the music and have appropriate track metadata (inc. Embedded album art).
While this means my soundtrack zip might not be password protected, I suspect that mp3tag has likely altered a file tag that prevents changes to the metadata, making the files look password protected and causing it to get flagged.
If it's nothing to do with the file, it could be because I've been using my login on pc and my phone, sometimes within moments of each other.
I suppose I would be interested to find out if any of my other projects are equally quarantined.
While looking around on this, it also appears as though itch support can be slow to respond, so I guess that's it for trying to do something good.