Thank you very much for engaging with this and your suggestions. I have figured out what the problem was: Postman requires you to save the Collection Folder after you update the Token value.
So I was changing the Token again and again but since I hadn’t saved, Postman was still providing a token from days ago. This token had obviously expired! (This has bitten me before, and honestly their model is totally confusing.) But now that I’m saving, the token is working on the backend both locally and in production. I wasn’t paying close enough attention to the value of the delivered token when I was trying to debug the API, or I would have caught this much sooner.
Hope my experience can help someone else in the future!