We can confirm this as well. But, it seems to be 1200 knots. Anything more than 1200knots closure speed, there's no radar return in TWS, once slowed down to below 1200, the return is back. Tested with an air started F14B, human RIO, opponent was human F16, but it's the same with an F18 as well. Attaching the track files of both the pilot and RIO. PS: This happens only with TWS. Pulse works fine.
Pilot.trk RIO.trk