Heroku says my custom domain is already in use, despite having already released it

Question

When I attempt to add a new custom domain to my heroku app, I receive the following error

Domain "..." could not be created:
... is currently in use by another app.

See this image: Example of error in the add new domain sidebar

Details so far:

1. I use a free dyno for my app
2. I own the domain and do not use it in any other heroku app (in fact, it's unused for any website)
3. I used this form, Heroku Domain Release, to release the domain from any existing usage on Heroku. This was successful and seems to imply it was in use by another app at some point.
4. I ran that form again, and received the message that it was no longer in use by any app.

It's been at least an hour since I tried using the release form and I still receive the same error message when trying to add my domain.

I cannot contact Heroku support directly since I use a free dyno.

Any help greatly appreciated!