If your only source for a version of the release is iTunes, you’re unlikely to learn what the UPC of the release is.
I import the vast majority of my digital media releases from stores where the record company can choose to be “open” about the UPC (e.g., Sony Music Entertainment Japan, Warner Music Group, and Universal Music Group on mora.jp). Then I could use the iTunes lookup API to see if there’s a match over there.
Note that it is expected that a “Mastered for iTunes” (16-bit AAC) release, a generic 16-bit lossy release, and a 24-bit release will each have a different UPC. (You’ll need to use the iTunes software to see if a release has the “Mastered for iTunes” sticker.) This is the primary reason you’ll often be unable to match a non-iTunes UPC from places like mora.jp and Spotify to an iTunes release. (Other reasons for different UPC between releases would be track counts, attachments like PDFs, or videos as part of the proper tracklist.)
Here’s an example of a release group where I filled out the barcodes of the digital media releases, in case you want to snoop around the edits to see what I did:
tl;dr - the “workaround” is to not use iTunes first.