Hi @jpayton-tes,
I confirmed with product manager that the URL of https://testorg.dev.ourdomain.com should work as it follows all rules listed here: Organization URL Placeholders
They recommended I open a up a ticket to the Engineering team for them to take a closer look at why this is not working for you. I will need a bit more information from you before I can do that. Can you kindly DM me the tenant name where you are implementing this, any relevant code/URL snippets, and capture a few screenshots showing the error?
Thanks,
Mary Beth