Oddity With OS X Content Filter Not Calling handleNewFlow

I'm working with content filters on OS X and encountered an issue where the handleNewFlow function is not called unless I explicitly invoke completionHandler(nil) in the startFilter method. This is perplexing because I expected handleNewFlow to be called automatically when new traffic flows are detected.

Here's an example of my startFilter function without the completionHandler:

override func startFilter(completionHandler: @escaping (Error?) -> Void) {
    os_log("Start filter called")
    
    // Create a network rule that matches all traffic
    let allTrafficNetworkRule = NENetworkRule(remoteNetwork: nil,
                                              remotePrefix: 0,
                                              localNetwork: nil,
                                              localPrefix: 0,
                                              protocol: .any,
                                              direction: .outbound)

    // Create a filter rule that allows all traffic
    let allowAllFilterRule = NEFilterRule(networkRule: allTrafficNetworkRule, action: .allow)
    
    // Set filter settings with the default action to allow all traffic
    let filterSettings = NEFilterSettings(rules: [allowAllFilterRule], defaultAction: .allow)

    // If I include this it will work fine, but TCP sessions will be reset:
   //completionHandler(nil)
}

Question:

Why, when I include completionHandler(nil) in the startFilter method, do all TCP sessions on my computer get reset? Is there a way to around this? My ideal state would be to get handleNewFlow to execute, but not without resetting sessions on my machine.