FilterUnloadCallback not called on exit

Hi, dears!

Just curiosity question!
I had the problem to unload the driver when it run in boot mode ( driver gone in on demand mode without problems), afterwards I found that in this mode one of my threads left in the air, and after finishing it, driver unloaded without problems.
During search I found that MSDN stand, that without FilterUnloadCallback set in FLT_REGISTRATION struct driver can’t be unloaded, which is not what happen to me, because my driver unloaded without it.
I added FilterUnloadCallback to FLT_REGISTRATION , but function never called on stop.
Maybe some can answer what’s the reason of minifilter not to call it.
I checked my filter struct with !fltkd.filter in windbg and see correct address of my
FilterUnloadCallback() in it.

TIA
Arkady

PS. Just forgot to mention that all other functions from the FLT_REGISTRATION called without problems