Azure Template with UI VirtualNetworkCombo hangs in "Loading..." - azure-resource-manager

Question

Created an Azure template with the Microsoft.Network.VirtualNetworkCombo type element. The new operation works fine. But two out of three existing Virtual Networks when selected show "Loading..." in each of the subnet dropdowns.
Screen snapshot
I cannot figure out why 2 of 3 existing Virtual Networks get stuck in the "Loading..." state. Does anyone know what would exhibit this behavior?
Update
It has something to do with adding IPv6 address space (10:8:2::/64). Virtual Networks/subnets without IPv6 address space display fine.
Perhaps it is something to do with the constraints minAddressPrefixSize "/29" specifications for the subnets?
I sure hope the answer isn't IPv6 is not supported...

Answer 1

I have tested in my environment it is working fine for me.
in createUiDefinition.json i have passed the below code in "steps": [ ]
{
"name": "NetoworkConfig",
"label": "NetworkSettings",
"subLabel": {
"preValidation": "Configure the storage settings",
"postValidation": "Done"
},
"bladeTitle": "Networking",
"elements": [
{
"name": "Configure Vnet Settings",
"type": "Microsoft.Network.VirtualNetworkCombo",
"label": {
"virtualNetwork": "Virtual network",
"subnets": "Subnets"
},
"toolTip": {
"virtualNetwork": "",
"subnets": ""
},
"defaultValue": {
"addressPrefixSize": "/16"
},
"constraints": {
"minAddressPrefixSize": "/16"
},
"options": {
"hideExisting": false
},
"subnets": {
"subnet1": {
"label": "First subnet",
"defaultValue": {
"addressPrefixSize": "/24"
},
"constraints": {
"minAddressPrefixSize": "/24",
"minAddressCount": 12,
"requireContiguousAddresses": true
}
},
"subnet2": {
"label": "Second subnet",
"defaultValue": {
"addressPrefixSize": "/26"
},
"constraints": {
"minAddressPrefixSize": "/26",
"minAddressCount": 8,
"requireContiguousAddresses": true
}
}
},
"visible": true
}
]
}
Output--
Reference : https://learn.microsoft.com/en-us/azure/azure-resource-manager/managed-applications/publish-service-catalog-app?tabs=azure-powershell