Ok, so I looked into this and the problem is more subtle. As stated above, the full descriptions are no longer in the Packages files but in the Translation-en file. What appears to be happening is that when a Package changes description and that this change doesn't propagate to all architectures at the same time, then there is a package in a Packages file that refers to a description that isn't present in the Translation-en file. That is the Translation-en only contains one description per package even though there are multiple versions in the archive.
I'm not entirely convinced this isn't a bug, but perhaps I'm missing something.