Hi Koji •,
Thanks for your patience.
I have tried to reproduce and could also not find option to select Availability Zone 1 from the dropdown while creation of Azure MySQL Flexible Server in Japan East region currently.
As per the official documentation here, it is mentioned that:
Zone redundant high availability is available in limited set of regions : Southeast Asia, WestUS 2, West Europe and East US.
Until the service expands to other regions too, we recommend to use above mentioned regions for instance East US:
We are sorry about the inconvenience you are facing and suggest you to raise Feedback over Azure MySQL Feeback Channel so that concerned team would look into the feature request and prioritize as per the importance of the and up-votes to implement it.
Hope this helps. Please let us know if further queries.
Thank you.