I couldn't test your original files, because you set the language code to French and I didn't want to uninstall my French dictionaries.
However, after changing the language codes and re-creating the .mobi files with KindleGen 2.8, the lookup worked fine in both lines:
Try changing the language codes. If that doesn't fix your problem, it's indeed a PW bug.
BTW, since older Kindles select dictionaries based on the <dc:Language> value of the ebook, you may want to use the proper language codes both for books and dictionaries. Though not strictly necessary, I'd also recommend using the same value for <dc:Language> and <DictionaryInLanguage> in dictionaries. This will allow non-native dictionary users to look up words used in definitions.