PAB came with a case in which one document was correctly showing a keywords while three were not, although they appeared identical. It turned out that the one that was working was doing so accidentally because elsewhere in it, there was a correctly-configured reference to the item, while the others had only the keyword entries which crucially were name elements lacking the @type attribute. Let PAB know; when she's fixed all such instances, I'll add a trap to the Schematron for this.