This is not a bug, though, but a feature.

As soon as you reveal the virtual keyboard, it installs a hook in the 
mapping kernel to take over control over some buttons exclusively, 
ignoring any mappings. The background is that this way you can map 
your button as you wish while the virtual keyboard is hidden - and 
then to use the virtual keyboard intuitively with the buttons when 
the virtual keyboard is being shown.

So, this is what happens

Button press => 
Virtual Keyboard (active) - directly filtering out left, right, up, 
down, A, B buttons => 
BehaviourCore handles the rest of buttons, executing the actions as 
mapped.

Button press => 
[Virtual Keyboard (inactive) - no filtering, no hooks] => 
BehaviourCore handles the all of the buttons, executing all actions 
as mapped.

Meanwhile, even programming an exception for the Wiimote's B button 
(which is easy as each action also knows which remote control the 
button press comes from) would not be a generally workable solution, 
as there are lots of users out there not using the IR mouse mode at 
all, who'd no longer be able to use the VK then.

I'll keep thinking about solutions, though.

Best regards, 
Felix