Um, not had time to analyse it yet, but I was having an similar issue on Saturday trip to Sleap. Visually spotted a mode S target about 2km and it was on SD, And audio alert heard, but only for a couple of seconds then it disaperaed off SD, despite the target passing down my port. Just like the transponder had been turned off. Didn't note if there was text.
Despite the great weather very little traffic seen, but was seeing stuff 50km away, so no issue with the ADSB rx.