HTTP 405 error after updating .NET 4.5.2 web api to .net 4.8

Question

I have a .NET 4.5.2 Web Api that I am converting to .NET 4.8.

I went into each project in the solution, and changed the target framework to 4.8.

I updated the nuget packages in the projects

One controller has two methods that are very similar:

public class InsuredController : ApiController
{
        [Route("thirdpartydata")]
        [HttpPost]

        public IHttpActionResult GetThirdPartyData([FromBody] ThirdPartyDataRequest request)
        {
            ...
        }

        [Route("masterinsuredlist")]
        [HttpPost]
        public IHttpActionResult GetMasterInsuredList([FromBody] MasterInsuredListRequest request)
        {
          ...
        }
    }


    public class ThirdPartyDataRequest
    {
        public string User { get; set; }
        public int AddressId { get; set; }
    }

    public class MasterInsuredListRequest
    {
        public string User { get; set; }
        public int AddressId { get; set; }
    }

When accessing the masterinsuredlist route, I get a result as expected.

However when accessing the thirdpartydata route, I get the error

HTTP/1.1 405 Method Not Allowed

I am using rest client to make the following call

POST http://localhost:123456/thirdpartydata HTTP/1.1
content-type: application/json
 
{
    "User": "rhodder",
    "AddressId" : 4163
}

Any ideas?