Seems to be a recent thing where the JAMBA Master keystrokes on switching windows are not being caught and are instead falling though to the game where F12 opens/closes a bag. If you don't normally use F12 to open/close the bag, you can remove that bind. The other option would be to either remove the Jamba Master mappe dkey action on switch (a setting on each slot in the character set), or to disable the JambaMaster mapped key in ISB42 - WoW, by shift+click on it so it is red.