Hi @jco - it's funny that recently there have been a couple of discussions here about keyboard shortcuts...
Anyway - I've tried here with PDF Expert and it all behaves as I (and you) would expect. Apart from triple-checking that you've got your key mappings right, then for troubleshooting, I'd point you towards two apps - KeyCue and ShortcutDetective which were mentioned in the previous discussions
Hope that helps.