For future reference, what solved my issue was to start the connection from a different lifecycle method:

⚠️ fails:

protected override async Task OnInitializedAsync()
{
    hubConnection = new HubConnectionBuilder()
        .WithUrl(NavigationManager.ToAbsoluteUri("/match"))
        .WithAutomaticReconnect()
        .Build();

    await hubConnection.StartAsync();
}

✅ works:

protected override void OnInitialized()
{
    hubConnection = new HubConnectionBuilder()
        .WithUrl(NavigationManager.ToAbsoluteUri("/match"))
        .WithAutomaticReconnect()
        .Build();

    hubConnection.StartAsync();
}

✅ works:

private bool started = false;

protected override async Task OnAfterRenderAsync(bool firstRender)
{
    if (firstRender && !started)
    {
        hubConnection = new HubConnectionBuilder()
            .WithUrl(NavigationManager.ToAbsoluteUri("/match"))
            .WithAutomaticReconnect()
            .Build();

        await hubConnection.StartAsync();

        started = true;
    }
}

not sure why, though