Slight reluctant to get too involved in non-PAW, but...
A screenshot of Richrads PAW traffic page could give as more info, as could Richards Log file for the test (we would also need the reg of the other aircraft)
When you say he was displayed as ADSB on SD, whose SD?
Can you confirm the FR24 track was not MLAT?
ATC are not involved in the exchange of data, so, yes you should have seen him.