Same problem here.
Mysql DB disappeared... suddently a morning.. without any action from me.
az mysql server show --resource-group myresourcegroup --name mydemoserver
says resource not found.
It's like DB never existed, but I still have the private network and dns associated with this DB (created from WordpressDB on Azure).
I still pay for the resource... Azure for Mysql Database appear in cost manager.
How is it possible ?!