swagger-client request body missing, content-length 0, and 415 response - .net-core

Question

I am maintaining someone else's web app (manual JavaScript files and MVC using swagger-js to connect to a .NET API).
The one POST route is returning 415 (Unsupported Media Type), but all the other GET routes work ok. I've had a look at the existing questions on this topic and they refer to incorrect specs or other issues which don't apply to me... I think.
You can quite clearly see from the Request Headers that nothing is posted in the body. But I don't know why:
POST /api/Trip/search HTTP/1.1
Accept-Encoding: gzip, deflate, br
Accept-Language: en-US,en;q=0.9
Connection: keep-alive
Content-Length: 0
Host: localhost:44393
Origin: https://localhost:44316
Referer: https://localhost:44316/
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: cors
Sec-Fetch-Site: same-site
User-Agent: Mozilla/5.0 (Windows NT 6.3; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/102.0.0.0 Safari/537.36
accept: application/json
sec-ch-ua: " Not A;Brand";v="99", "Chromium";v="102", "Google Chrome";v="102"
sec-ch-ua-mobile: ?0
sec-ch-ua-platform: "Windows"
The Swagger spec is generated from dotnet core 3.1 using this route:
[Route("search")]
[HttpPost]
[ProducesResponseType(StatusCodes.Status200OK)]
[ProducesResponseType(StatusCodes.Status400BadRequest)]
public async Task<ActionResult<TripInformationSearchResponse>> TripInformationSearch(TripInformationSearchRequest request)
{
...
}
Part of the generated spec is:
"/api/Trip/search": {
"post": {
"tags": [
"Trip"
],
"operationId": "TripInformationSearch",
"requestBody": {
"content": {
"application/json": {
"schema": {
"$ref": "#/components/schemas/Cust.API.Models.Trip.TripInformationSearchRequest"
}
},
"text/json": {
"schema": {
"$ref": "#/components/schemas/Cust.API.Models.Trip.TripInformationSearchRequest"
}
},
"application/*+json": {
"schema": {
"$ref": "#/components/schemas/Cust.API.Models.Trip.TripInformationSearchRequest"
}
}
}
},
"responses": {
"200": {
"description": "Success",
"content": {
"application/json": {
"schema": {
"$ref": "#/components/schemas/Cust.API.Models.Trip.TripInformationSearchResponse"
}
}
}
},
"400": {
"description": "Bad Request",
"content": {
"application/json": {
"schema": {
"$ref": "#/components/schemas/Microsoft.AspNetCore.Mvc.ProblemDetails"
}
}
}
}
}
}
}
The webapp client uses swagger-js from JavaScript (not exactly how I would do it but I'm not about to rewrite it just now) to call the search_trips function which calls the above API route:
CUSTAPI = {
swagger_client: function (urlBase) {
var specUrl = urlBase + '/swagger/v1/swagger.json';
SwaggerClient.http.withCredentials = true; // this activates CORS, if necessary
var swaggerClient = new SwaggerClient(specUrl);
return swaggerClient;
},
// ...and later
search_trips: function (keywords, tripNumbersArr, status, routeType, hasGLPostDate, successCallback) {
var failedSwaggerLoadSpecCallback = function (reason) { CUSTAPI.error_display("Unable to connect to the API: " + reason); };
var failedApiRequestCallback = function (reason) { CUSTAPI.error_display("Unable to fulfill the API request: " + reason); };
CUSTAPI.swagger_client(oldApiClient_UrlBase)
.then(
function (swaggerClient) {
var searchRequest = {
keywords: keywords,
tripNumbers: tripNumbersArr, //array
status: status,
routeType: routeType,
hasGLPostDate: hasGLPostDate
};
return swaggerClient.apis.Trip.TripInformationSearch(searchRequest); // chaining promises
}, failedSwaggerLoadSpecCallback)
.then(function (response) {
if (response.ok)
successCallback(response.obj);
//else
// return response
}, failedApiRequestCallback);
},
...
}
And from there no amount of debugging can show me why nothing is put in the body. Can you see why?