[application-gateway] Not possible to update backendIpConfigurations of an address pool

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**Describe the bug**
I can not update the backendIpConfigurations of an application-gateway via this command.
az network application-gateway address-pool update --subscription -g things-resources-int --gateway-name -n --add backendIpConfigurations "{\"id\":\"/subscriptions//resourceGroups//providers/Microsoft.Compute/virtualMachineScaleSets//virtualMachines/0/networkInterfaces//ipConfigurations/ipconfig1\", \"resourceGroup\": \"\" }"

The command is running for about 30s and afterwards no changes are visible to the address pool.

**To Reproduce**
Run the previous mentioned command and try to update the backendIpConfigurations.

**Expected behavior**
The address-pool is updated with the specified json value.

**Environment summary**
az version
{
"azure-cli": "2.26.1",
"azure-cli-core": "2.26.1",
"azure-cli-telemetry": "1.0.6",
"extensions": {
"azure-iot": "0.10.13"
}
}