Signal-r does not work with devices on IPv6 (- works as expected on IPv4)

I am using Microsoft.AspNetCore.SignalR.Client in a .Net Maui app

Calling

await HubConnection.StartAsync(); on some devices (Android, Windows tablet, Windows PC) - everything proceeds to work fine - the connection is open and I can send and receive messages. However on some devices StartAsync repeatedly throws:

System.Net.Http.HttpRequestException: Response status code does not indicate success: 403 (Forbidden).

What is the difference between the devices where it works and device where it doesn't? Devices where it works are assigned IPv4 addresses, but devices that throw 403 Forbidden are IPv6. Adding a local proxy and forcing the signal-r traffic through it to ensure IP is IPv4 resolves issues.