为了回答我自己的问题，这里有一个可能的解决方法：

services
        .AddMvc(config =>
            {
                ...
                config.OutputFormatters.Add(new CustomXmlOutputFormatter());
                config.RespectBrowserAcceptHeader = true;
            });

public class CustomXmlOutputFormatter : TextOutputFormatter
    {
        public CustomXmlOutputFormatter()
        {
            SupportedMediaTypes.Add(MediaTypeHeaderValue.Parse("application/xml"));
            SupportedMediaTypes.Add(MediaTypeHeaderValue.Parse("text/xml"));

            SupportedEncodings.Add(Encoding.UTF8);
            SupportedEncodings.Add(Encoding.Unicode);
        }

        protected override bool CanWriteType(Type type)
        {
            return true;
        }

        public override async Task WriteResponseBodyAsync(OutputFormatterWriteContext context, Encoding selectedEncoding)
        {
            if (context == null) throw new ArgumentNullException(nameof(context));
            if (selectedEncoding == null) throw new ArgumentNullException(nameof(selectedEncoding));

            var httpContext = context.HttpContext;

            var json = JsonConvert.SerializeObject(new { item = context.Object });

            var xml = JsonConvert.DeserializeXNode(json, "root");

            var buffer = new StringBuilder(xml.ToString());

            await httpContext.Response.WriteAsync(buffer.ToString());
        }
    }