when inside the HTML area of Safari, VoiceOver nearly always fails to toggle Quick Nav with the left and right arrows. It seems like the arrows are captured as commands to move by character, even though they are pressed simultaneously. Enabling Quick Nav in other apps works as expected, thus letting you use the workaround of toggling it outside of Safari. It has been pointed out that pressing Caps Lock when it is set as a VO modifier will cause this problem, while pressing it twice to turn caps lock on or off then lets you use quick nav as normal.
Bug First Encountered
How often the bug occurs
It appears that pressing the control key before toggling Quick Nav will cause the command to work normally, though we aren't sure if that's a definite fix/work around or not.
I have vo modifiers set to control+option and capslock (el cap defaults), and quick nav turns on normally for me in safari.