Troubleshooting model binding problem in ASP.NET Core 3.1 API - asp.net

Question

I'm trying to send an object via a POST request to my ASP.NET Core 3.1 API but I keep getting Bad Request error. As far as I can see, I do have a class that matches what I'm expecting perfectly but clearly it's not. How can I see exactly what the problem is?
The following fails:
public async Task<IActionResult> Post([FromBody] MyCustomObject input)
{
// Do something here...
}
If I use a dynamic, it works fine. So the following code works fine:
public async Task<IActionResult> Post([FromBody] dynamic input)
{
// Do something here...
}
As I said, I'm just getting a 400, Bad Request error. I've been going over MyCustomObject again and again and it looks identical to the object that I'm sending.
Here's what my custom class looks like:
public class CreateContactVm
{
[GuidEmptyNotAllowed]
public Guid AccountId { get; set; }
[Required]
public string AccountName { get; set; }
[GuidEmptyNotAllowed]
public Guid ContactGroupId { get; set; }
[IntZeroNotAllowed]
public int ContactType { get; set; }
public string FirstName { get; set; }
public string MiddleName { get; set; }
public string LastName { get; set; }
public string EntityName { get; set; }
public List<AddEmailVm> Emails { get; set; } = new List<AddEmailVm>();
public List<AddPhoneNumberVm> PhoneNumbers { get; set; } = new List<AddPhoneNumberVm>();
public List<AddAddressVm> Locations { get; set; } = new List<AddAddressVm>();
}
Here, I use some custom validations such as [GuidEmptyNotAllowed] or [IntZeroNotAllowed]. I inspect the object I send via my POST call and it satisfies ALL of these requirements and yet it still fails.
How can I get more information about why my API method is throwing a 400 error?
UPDATE:
The following code allows me to convert what comes in as a dynamic to my CreateContactVm custom class but I really shouldn't have to do this at all:
CreateContactVm request = new CreateContactVm();
try
{
var element = (JsonElement)input; // input is the dynamic received
request = JsonUtils.Deserialize<CreateContactVm>(element.GetRawText());
}
catch(Exception e)
{
var error = e.Message;
}
This also proves that the issue is with model binding. Something in my custom class is not liking the JSON object it receives.