You're welcome.
Fastboot is not a true clean/fresh boot.
It skips things and that makes avast believe the system wasn't turned off (which is correct).
Let's say you need to clean a house.
Washing the windows, emptying the trash bins, vacuum clean the floors etc.
You are working as fast as you can but it will take a certain amount of time.
It is not possible to do it faster.
So how do you safe time?
Only way is to skip things and that is what fastboot is doing.