Hi Esteban, that's odd.
The fixture black out works fine for us, and up until now we have never heard of such an issue.
If you manually play with the shutter channel, you will see that the shutter closes really fast.. the shutter can't dim.
So what I think is happening is that when you activate the blackout function, it is for some reason using the dimmer channel instead of the shutter channel.
Blackout should be directed to the shutter.
Give these things a try.. I hope it works for you.
Best regards,
JL