Upon further investigation, it appears that I was wrong. Zortam does in fact add the mp3 to the list, albeit with the errors in the tags. I was able to correct all of the bad tags (I think), so unfortunately I probably no longer have any that I can send you that still have the errors.
I’m sure that this is related to the previous issue I had with bad tags due to Zortam not being able to read UTF-16 Unicode tags with the previous version. It appears that Zortam had corrupted some of the tags at that time and was unable to read all of the files that had previously been corrupted in this way. It read most of them, just not all.
This bug seems to be “fixed” and is only presenting itself as a result of the prior damage that was done by the bugged version(s) of Zortam.