Was fighting some ships for a quest when I looked through my backpack and realized that you can't delete things while in broadside combat. It works fine when you're just sailing around, but not in ship combat. Is this a bug?
No, you cannot interact with your bank or backpack during ship combat, because it won't allow you to change ship equipment while fighting. As you say, you can sail to a windlane to do your inventory management and then get back into your nautical battle afterward.
Was fighting some ships for a quest when I looked through my backpack and realized that you can't delete things while in broadside combat. It works fine when you're just sailing around, but not in ship combat. Is this a bug?
Not a bug, it's there to prevent you from doing that in combat, You're supposed to be concentrating on what you're doing!