Azurerm postgresql flexible server not possible to provision in singe zone HA

When deploy flexible postgresql server in HA mode, as per terraform docs it’s only possible to provision ZoneRedundant HA mode. However, in Azure there is option to provision single zone HA which seems is not possible to be done with terraform because there is no other mode beside ZoneRedundant available.
This becomes to be very important in single zone Azure regions, as not possible to provision HA DB.
Would you please advice if there is possibility to provision single zone HA flexible postgfresql server? Is this a bug? What is reason why single zone HA is not enabled in terraform?