In "CharacterUtilite.c", function 'GetSquadronGoods' skips any ship marked as unremovable, as these are quest ships. But 'AddSquadronGoods' and 'TakeSquadronGoods' do not. So, if your own ship does not have enough space for quest cargo, and you have a quest companion and a non-quest companion, the check shows that you have enough space and then some of the cargo goes into the quest ship. This happened to me when I was doing the "Silver Train" quest, Will Turner's ship was in slot 2, I had another ship in slot 3, then I took "Cargo for Thomas O'Reily". Some of Thomas' cargo went into Will's ship, then Arnaud Matton couldn't see it.
The attached version of "CharacterUtilite.c" adds the check to "AddSquadronGoods" and "TakeSquadronGoods", so no quest goods are placed in quest ships.
The attached version of "CharacterUtilite.c" adds the check to "AddSquadronGoods" and "TakeSquadronGoods", so no quest goods are placed in quest ships.