The small jet could have had a Garmin 330 transponder .
This mode S transponder (/current software version) has a problem with TCAS interrogations, but not with SSR.
There is an SB out for this problem, but I think this might answer to the question without opening the box (TCAS)
Jaap
("old" Cirrus SR20 owner (2003) without PFD but with MFD and all other bells and w's)