You can choose how Breww should predict the sales with the “Sales forecast method” option in the form and one of these options is to just use your outstanding deliveries. The default option will look at recent sales and make a prediction of typical sales based upon those, but it tries not to be skewed too much by a one-off big order and so might not include a one-off order (fully), which I know looks strange if you know you have it, but it’s trying to predict the normal sales rate.
There’s also Manually set expected sales rate in Stock availability forecast as an outstanding request that you might find useful and want to vote for?