When GARNIN GPS unit lost synchronization, it starts to send GPRMC sentence without time. And Mikrotik Router OS use time from the last valid GPRMC sentence again and again that results in incorrect time. Tested with 2.9.43.
Example of GPRMC when position was lost:
$GPRMC,V,150607,2.3,E,N*3C
More details can be found here:
http://forum.routerboard.com/viewtopic.php?f=3&t=3598