Sorry, that's not something we can reproduce. Any displayed information is not cached or stored in a database, so a rescan should always fetch the latest data.
Here, I have scanned a NAS device that had a number of recorded discards on its port eth0:
Then, I rebooted the device to clear the counters, rescanned it again, and the counters have been reset:
Perhaps you could try restarting the Port Mapper application and/or the switch to see if that changes anything?