Getting ArgumentException: Invalid type parameter using ActionResult<JsonResult>> as a return type after upgrading to .NET Core 2.2

I'm getting the following runtime error when trying to hit any of my API endpoints that are built like the code seen below:

ArgumentException: Invalid type parameter 
'Microsoft.AspNetCore.Mvc.JsonResult' specified for 'ActionResult<T>'.

This code functioned as expected in my app in .NET Core 2.1. After upgrading to .NET Core 2.2, this broke.

Here's the code for one of me endpoints in a controller.

    [HttpGet]
    public async Task<ActionResult<JsonResult>> Get()
    {
        try
        {
            MongoConnector mongo = new 
            MongoConnector(_configService.GetMongoConnectionString());

            List<BsonDocument> queues = await mongo.AggregateQueues(
                (aggregator) => aggregator
                                .Match(CommonAggregation.SinceYesterday)
                                .Sort(CommonAggregation.SortByDate)
                                .Group(QueueAggregators.GroupQueuesByMessagingName)
            );


            List<QueueHealth> queueHealth = await MongoConnector.DeserializeList<QueueHealth>(queues);
            JsonResult result = new JsonResult(queueHealth);
            result.Value = queueHealth;
            result.ContentType = "application/json";
            result.StatusCode = 200;

            return result;

        }
        catch (Exception ex)
        {
            logger.Error(ex, "An exception was thrown within /api/MessageQueue");

            return new StatusCodeResult(InternalServerError);
        }
    }

Anyone have any insight on what changed between 2.1 and 2.2 that would cause this? I can't even find anyone else mentioning a similar issue or documentation on the change online.