Angular service error http 400 using .Net core api to accept json, escaped quotes crashing api

I spent all day on this. It just started happening recently.
So I package a model in Angular, and create a body of JSON using JSON.stringify(model)
Then package it all up and send it as a promise. I just noticed that the JSON quotes are escaped out with a slash, and it crashes the API. If I take the slashes out, the API accepts the JSON.

I'm not sure what to make of this, and if I should fix the client side or work on the server side with this.
I put the replace(/\/g, "") in below to test the theory that it's the slashes crashing the API. Now the API works, but I can't see this as a permanent fix.

validateTokenAsPromise(signIn: SignIn): Promise<SignIn> {
    const authUrl = this.baseUrl + "api/auth/AuthenticateAccount";
    const toSignIn = JSON.stringify(signIn).replace(/\\/g, "");    
    const httpOptions = {
      headers: new HttpHeaders({
        "Content-Type": "application/json",
        "Accept": "application/json"
      })
    };
    return this.http.post<SignIn>(authUrl, toSignIn, httpOptions).toPromise();
  }

I send it to a .Net Core controller api, and the api rejects the call, because of the data in the body.
The data has the quotes escaped out.

[Produces("application/json")]
[Route("api/[controller]")]
[ApiController]

[HttpPost("AuthenticateAccount")]
public async Task<SignIn> AuthenticateAccount([FromBody] SignIn authParam)
{

Sending this data to the API, normally it just sends raw, but I wrote a special service to handle Google SignIn and Googles gapi and gapi.auth2. I can't see how this special API" would encode the JSON.

{\"Id\":\"114251204162343478403\",\"FirstName\":\"John\",\"LastName\":\"Smith\",\"AccountName\":\"jsmith@hackme.com\",\"Password\":\"2H3HY4ZwE8Sb5Ajz\",\"Token\":\"eyJhbGciOiJIUsdadasdasd1lIjoiSmltIiwiZW1haWwiOiJqa2lya2VyeEBnbWFpbC5jb20iLCJuYmYiOjE1OTM0NjkwMDYsImV4cCI6MTU5MzY0MTgwNiwiaWF0IjoxNTkzNDY5MDA2LCJpc3MiOiJodHRwczovL2xvY2FsaG9zdDo1MDAxLyIsImF1ZCI6asdsadasdasdasdwMS9hcGkifQ.WdpYg8qDIUtIT4uobyAWVQ01k9iNYJ6wPpa6FIXm3Yg\",\"Role\":\"Customer\",\"ExpiresAt\":\"2020-06-29T23:26:03.812Z\",\"IdpId\":\"gapi\",\"RememberMe\":true,\"Avatar\":{\"Url\":\"https://lh3.googleusercontent.com/a-/AOasdasdasdGJB_aUO-asdasdasdg-ob=s96-c\"}}

Don't get excited about the data, it's fake.