Find my drone works by the Mavic providing the GPS coordinates to the app whenever there is a solid signal between RC and Mavic. Your signal was lost just before RTH and was not restored so no updated GPS was provided after the loss. You had the video link restored for that one or two seconds after the crash but that is not the same as the RC signal. Could only speculate about the OA - the sensors might have had trouble "seeing" that shaded and non-descript area where it crashed.....