Zoho Writer document save not working no error

Question

I'm working on integrating Zoho Writer into my web app using Zoho Office Integrator. My web app uses an WebAPI api in c#, .net framework 4.8 and the zoi-csharp-sdk. I am able to open a file from local storage by following the Zoho docs and code examples. The code that opens the doc includes a bit that sets the Save parameters as follows:

using Com.Zoho.Util;
using Com.Zoho.Officeintegrator.V1;
using Com.Zoho;
using Com.Zoho.Dc;
using Com.Zoho.API.Authenticator;
using Com.Zoho.API.Logger;
using static Com.Zoho.API.Logger.Logger;

...

    Dictionary<string, object> saveUrlParams = new Dictionary<string, object>();
    saveUrlParams.Add("templateId", templateId);
    saveUrlParams.Add("filePath", networkPath);
    Dictionary<string, object> saveUrlHeaders = new Dictionary<string, object>();
    saveUrlHeaders.Add("Authorization", _context.Token);
    CallbackSettings callbackSettings = new CallbackSettings();
    callbackSettings.Retries = 2;
    callbackSettings.Timeout = 10000;
    callbackSettings.SaveFormat = "docx";
    callbackSettings.HttpMethodType = "post";
    callbackSettings.SaveUrlParams = saveUrlParams;
    callbackSettings.SaveUrlHeaders = saveUrlHeaders;
    if (APIBaseURL.IndexOf("localhost") > -1) APIBaseURL = "https://my url";
    callbackSettings.SaveUrl = APIBaseURL + "api/files/save-zoho-doc";
    createDocumentParams.CallbackSettings = callbackSettings;

I have checked that APIBaseURL is accessible from the internet. When I click the Save option in the Writer editor nothing happens. No errors and no logs. The breakpoint in the api call save-zoho-doc is never hit. Here is an excerpt from the api

    [HttpPost]
    [Route("save-zoho-doc")]
    [SwaggerResponse(HttpStatusCode.OK, typeof(string))]
    public IHttpActionResult SaveZohoDoc([FromUri] int templateId, string filePath) {

        System.Web.HttpRequest httpRequest = System.Web.HttpContext.Current.Request;
        if (httpRequest.Files.Count > 0 && !string.IsNullOrEmpty(filePath)) {
    ...