Welcome to FT !

Yes, this is correct at time of booking the miles are deducted from your balance and show on your account as an award booking. When you do take the flight the miles appear on your statement again showing a deduction but this time they do not affect the balance.

This was what happened in my case earlier this year anyway with an FD award with the flight taken after the start of FB.

Unsure if this happens with FB awards aswell as have not taken an award since.