Subscribing to an eventhandler doesn't work in abstract class using dependency injection in C# .NET 6

I have an abstract class WebsocketHandler which is inherited by WsUserRequestHandler which implements an interface which in turn is injected as a singleton service to be used in other classes.

WebsocketHandler (simplified):

public abstract class WebSocketHandler
{
    private event EventHandler<OnRequestReceivedEventArgs> OnRequestReceived;
    private ClientWebSocket _client { get; set; } = new ClientWebSocket();

    protected async Task<bool> Connect(WsConnectionInfo connectionInfo)
    {
        this._client.ConnectAsync(...)
        
        while (true)
        {
            await this.Receive(this._client);
            if (this._cts.IsCancellationRequested)
                break;
        }
    }

    private async Task Receive(ClientWebSocket client)
    {
        while (true)
        {
            var result = await client.ReceiveAsync(...);
            if (result.MessageType == WebSocketMessageType.Text)
            {
                var jsonInput = Encoding.UTF8.GetString(...);
                var request = JsonConvert.DeserializeObject<IWebSocketRequest>(jsonInput, ...);

                if (request is null)
                    continue;

                // Always true
                Console.WriteLine("received request event is null: " + (this.OnRequestReceived == null));

                // Always null, so not being invoked
                this.OnRequestReceived?.Invoke(this, new OnRequestReceivedEventArgs(request)); 
            }
        }
    }

    // this method runs as expected when called and ReceivedRequest gets 
    // successfully attached to event handler (at least in the scope of the method)
    protected async Task<TRequest> RequestReceived<TRequest>(CancellationToken token) 
        where TRequest : IWebSocketRequest
    {
        var request = default(dynamic);

        void ReceivedRequest(object sender, OnRequestReceivedEventArgs args)
        {
            if (args.Request.GetType() == typeof(TRequest))
                request = args.Request;
        }

        this.OnRequestReceived += ReceivedRequest;
        Console.WriteLine(this.OnRequestReceived == null); // returns false

        while (request == null)
            await Task.Delay(25, token);

        this.OnRequestReceived -= ReceivedRequest;

        return request;
    }

The class WsUserRequestHandler is injected into another class which then calls the base method RequestReceived and awaits it. If I then, after making sure that the method is called and awaited, send a message to the websocket, the Receive method is called and OnRequestReceived turns out null. I have also made sure that the cancellation token that gets passed on to RequestReceived isn't cancelled, and also that it doesn't unsubscribe from the event.