this has been going on for years and years. it has been my experience involved in international freight and losses of this nature that its typically SLC (Shipper Load Stow and Count) containers that are the culprits. as the duties and fees are much less if you declare the cargo something low value. Then the shipping costs are lower rather than something high value and or possibly volatile. In the US they only spot check X ray some containers and not all.
it bad for the ship as it effects the stow plan and you wind up with these dangerous containers shipped low in the ship as opposed to top stow as outboard as possible where it can be jettisoned.
as a side bar the lazy ass stevedores fail to secure and unitize the containers properly leading to container falling overboard or container stack collapse during the voyage.
not sure what the solution is as many times the affected area of the fire is so deteriorated that it is difficult to tell what was in the container and they often melt together, and there is danger to the investigators trying to determine what may have transpired. during removal the damaged cargos are often comingled at that point making the origin hard to determine.
modern container ships are open hatch. perhaps the answer is some kind of a water type fire system attached to the cells so that the fire can be put out automatically.