Having a read of the Universal user manual for the unit I use interestingly says:
During enroute and approach operations, the FMS provides pilot- requested predictions of RAIM availability at the selected ARRIVE waypoint for a period of ETA ±15 minutes in five minute increments.
NOTE: The FMS does not provide automatic RAIM prediction at approach arming and activation.
Hense why it would be prudent to conduct one in flight prior to approach.