I don't know about Britannia ships but the very same thing happens on Tokuno ships too. The thing is: Tokuno ship's cargo hold artwork consists of 6 separate tiles like this:
x x x
x o x
and only the tile "o" actually displays the contents of the cargo hold while others display blank boxes. This is also the case for the tile just under the cannons. Some of them are special tiles (cargo hold ones can be used to open the cargo hold, the others are used when placing cannons) but the main mast of the ship also displays a blank box.
I think it is a bug.