I, too, found that problem when the automatic choice of behaviours 
for fore- and background applications is disabled. Are (or have) 
those by any chance (been) your settings, too?

I am currently spending a lot of time on the internals that do choose 
behaviours, interact with the menu and issue a beep if no 
functionality was found for a button press. In fact, I took the 
opportunity to clean up these parts once and for all and replaced 
them entirely. Gives some bonus functionality, too, but more on this 
on preview 6 release day ;-)

Anyway, back on topic: with the changes I made, this problem is gone 
here.

Best regards, 
Felix