I have the same problem.It's still doing this really weird thing in single target which you really need to fix, as it's a 2-3k DPS loss, believe it or not.
If you end up with 10 or so focus and Explosive Shot is off cooldown, it WAITS and does NOTHING until you have 15 Focus so it can cast Explosive Shot, then it does the Focusing Shot (which it should've done straight away instead of waiting for the 5 additional focus for ES).