When the option for bookable itemtypes was added, we expanded the values for the bookable column in items from 0,1 to 0,1,NULL Existing items would have been at 0 - this means that if an itemtype is set to bookable, individual items won't follow that setting. The upgrade to add that option to the column should also set the current values to NULL. It won't change behaviour, but it will allow the new option to take effect.