Azure – Conditional object construction not working ARM Template

I’m trying to construct an Azure ServiceBus Subscription Rule that will have either a correlation filter or sql filter depending on the length of param’s subjectFilter array property. I tried this using if with union, based on this answer. However, it looks like the object is constructed with all properties. For example, the sql filter errors with "At least one system or user property must be set for a correlation filter."

Here’s my code (simplified).

• The copy variable contains properties to use for each subscription name and filterProperties, with defines a filterType and is used as the base properties object.
• It also declares a separate object for the sqlFilterProperties and correlationFilterProperties, respectively.
• Either the sqlFilterProperties or correlationFilterProperties object should be unioned with the filterProperties to construct the complete properties.

{
    "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
    "contentVersion": "1.0.0.0",
    "parameters": {
        "resourceName": {
            "type": "string"
        },
        "topics": {
            "type": "array"
        },
        "subscriptionName": {
            "type": "string"
        }
    },
    "variables": {
        "apiVersion": "2018-01-01-preview",
        "copy": [
            {
                "name": "subscriptions",
                "count": "[length(parameters('topics'))]",
                "input": {
                    "name": "[concat(parameters('resourceName'), '/', parameters('topics')[copyIndex('subscriptions')].name, '/', parameters('subscriptionName'))]",
                    "filterProperties": {
                        "filterType": "[if(greater(length(parameters('topics')[copyIndex('subscriptions')].subjectFilter),1),'sqlFilter','correlationFilter')]"
                    },
                    "sqlFilterProperties": {
                        "sqlFilter": {
                            "sqlExpression": "[concat('sys.subject in (''',join(parameters('topics')[copyIndex('subscriptions')].subjectFilter,''','''), ''')')]",
                            "requiresPreprocessing": false
                        }
                    },
                    "correlationFilterProperties": {
                        "correlationFilter": {
                            "subject": "[parameters('topics')[copyIndex('subscriptions')].subjectFilter[0]]",
                            "requiresPreprocessing": false
                        }
                    }
                }
            }
        ]
    },
    "resources": [
        {
            "type": "Microsoft.ServiceBus/namespaces/topics/subscriptions/rules",
            "apiVersion": "[variables('apiVersion')]",
            "copy": {
                "name": "subscriptionFilters",
                "count": "[length(parameters('topics'))]"
            },
            "name": "[concat(variables('subscriptions')[copyIndex()].name, '/subject')]",
            "properties": "[union(variables('subscriptions')[copyIndex()].filterProperties, if(startsWith(variables('subscriptions')[copyIndex()].filterProperties.filterType, 'sql'), variables('subscriptions')[copyIndex()].sqlFilterProperties, variables('subscriptions')[copyIndex()].correlationFilterProperties) )]"
        }
    ]
}