I seen / heard the same today.
At 3,600ft, audio alerts were giving traffic at 1000ft below, but mode S ring was showing the traffic at -3,600ft.
The other AC appeared to be 'following' us and as per audio alerts climbing to within 300ft our alt.
Heads spinning and eyes on stalks, couldn't see the other ac for about 8 mins, starting to think sally was telling fibs / wierd bug, but she was being persistent.
THEN, out of the right window, below and behind, there it was, and crossed underneath us.
The FlowerPlarm we also carry was late in giving the warning.
I have various screen grabs of SD nav page, SD track log, PAW traffic page and FR 24 flight path of the other AC. (Post these once collated).
FR24 track explains whey we kept getting the alerts for so long a period.
PAW was worth every penny today.
Interesting to note, our mode c transponder doesn't show as SELF in the traffic page. Checked several times today.
Deker