Not a bug, the policy states "The police department can recruit up to seven tactical firearms officers". The max amount of TFU slots depends on the total amount of police on duty at any given time. So this policy just allows you to recruit up to seven tfu officers but does not guarantee you can. If you reach like 12-13 police officers, you will be able to have seven tfu officers. If this is something you feel we should change, feel free to make a suggestion
And as a note, I have tasked @Efan with coming up with a better description for the policy so it's more clear.