Wrong HttpParams generation for { [key: string]: Array<string>; }

By default AspNetCore supports parameters either of type IDictionary<string, string> or even IDictionary<string, IEnumerable<string>> (e.g. for queries like /get?id=1&id=2).

Technically this is perfectly correct and swagger-codegen (at least typescript-angular) supports the first scenario:

order?: Array<string>

if (order) {
    order.forEach((element) => {
        queryParameters = queryParameters.append('order', <any>element);
    })
}

but seams to have issues with the second one:

paramProperties?: { [key: string]: Array<string>; }

if (paramProperties !== undefined && paramProperties !== null) {
    queryParameters = queryParameters.set('paramProperties', <any>paramProperties);
}

This way a serialized objects gets added to the HttpParams.

Start by locating the typescript-angular generation logic for dictionary-valued query parameters and compare it with the shown Array handling. Verify generated HttpParams for paramProperties: { [key: string]: Array } preserves repeated query values instead of serializing the object.