Unmatchable ticket because no valid azure regions...but I'm using an azure region

I'm getting an error response after calling GetMatchmakingTicket()

A combination of the ticket parameters and the rules for the queue 
makes this ticket unmatchable. None of the common regions across the 
users in the ticket are valid azure regions.

However, I'm using the EastUs region, which seems to be a valid Azure region. (reference:

https://docs.microsoft.com/en-us/gaming/playfab/api-references/events/data-types/azureregion)

My TitleID is B186.

Happy to provide full source code for my client if it's helpful, since I'm running this all on a test project while I learn the framework.