Shutdown of QNX with a flash file system

We are running a flash file system undex QNX6.1.0 and everything works very
well. When we want to reboot the system we use the standard command
Once in a while (maybe 1 out of 20) one of our four flash partitions are
corrupted when the system starts again after the shutdown.
The power/voltage has been on all the time.

My question is: shall the flash file system drivers (devf-…) be
shutdown/terminated in a special way to avoid this?

Can the flash file system become corrupted during normal operation in such a
way that it is not discovered untill the next startup of the drivers?

Best regards, Mats B