I'm replacing some icons & tokens I dislike with custom ones.
In trying to solve this, I figured the simplest thing to do would be to unzip re-zip and see if it worked having changed nothing inside the archive.
I tried all compression levels and couldn't match the original file size. I left that out earlier because I hadn't noticed at the time the file sizes were mismatched.
One other thing, the campaign loads fine if left as a folder rather then archiving to .pak. This is why I've concluded I'm doing something wrong.