System.Net.HttpClient throws COMException when attempting to access internet from corporate domain

I am writing my first UWP app (for Windows 10 only), using Xamarin.Forms. I have written the following test class to establish that I can connect to the internet from my app:

public class NetworkInterface
    private static readonly System.Net.Http.HttpClient WebClient = new HttpClient();

    private void Get()
            var response = WebClient.GetAsync($"").Result;
        catch (Exception e)
            Debug.WriteLine("Error: " + e);

I would expect that I would receive a 404 response to this query, but instead I receive the following:

System.Net.Http.HttpRequestException: An error occurred while sending the request. ---> System.Runtime.InteropServices.COMException: The text associated with this error code could not be found. A connection with the server could not be established

I know this question has been posed a few times before, and that the answer is typically that the author has not enabled certain UWP Capabilities. I have attempted to remove that as a potential barrier by enabling all available Capabilities. From my Package.appxmanifest:

    <Capability Name="internetClient" />
    <Capability Name="internetClientServer" />
    <Capability Name="privateNetworkClientServer" />
    <Capability Name="codeGeneration" />
    <Capability Name="allJoyn" />
    <uap:Capability Name="enterpriseAuthentication" />
    <uap4:Capability Name="userDataTasks" />
    <uap:Capability Name="voipCall" />
    <uap:Capability Name="videosLibrary" />
    <uap3:Capability Name="userNotificationListener" />
    <uap:Capability Name="userAccountInformation" />
    <iot:Capability Name="systemManagement" />
    <uap2:Capability Name="spatialPerception" />
    <uap:Capability Name="sharedUserCertificates" />
    <uap:Capability Name="removableStorage" />
    <uap3:Capability Name="remoteSystem" />
    <mobile:Capability Name="recordedCallsFolder" />
    <uap:Capability Name="picturesLibrary" />
    <uap2:Capability Name="phoneCallHistoryPublic" />
    <uap:Capability Name="phoneCall" />
    <uap4:Capability Name="offlineMapsManagement" />
    <uap:Capability Name="objects3D" />
    <uap:Capability Name="musicLibrary" />
    <iot:Capability Name="lowLevelDevices" />
    <uap:Capability Name="contacts" />
    <uap:Capability Name="chat" />
    <uap:Capability Name="blockedChatMessages" />
    <uap:Capability Name="appointments" />
    <uap3:Capability Name="backgroundMediaPlayback" />
    <uap6:Capability Name="graphicsCapture" />
    <DeviceCapability Name="webcam" />
    <DeviceCapability Name="proximity" />
    <DeviceCapability Name="pointOfService" />
    <DeviceCapability Name="microphone" />
    <DeviceCapability Name="location" />
    <DeviceCapability Name="gazeInput" />
    <DeviceCapability Name="bluetooth" />

However, it is my understanding that I should only need the "internetClient" Capability for my intended use of accessing APIs over the internet.

Here are some additional wrinkles:

  1. I successfully receive a 404 when I disable my ethernet connection and connect to wifi. If I connect to VPN over wifi back into my domain I get the exception again.
  2. Running the same query in Postman (even against the real API I'm trying to access) works regardless of network/domain connection.

My hunch is that UWP is lacking some sort of authorization or capability I don't know about for my domain network. Thanks for any advice on this!