Which character? Does this happen with the left/right-click? Mouseover? Other mouse button?

You say there is sometimes a sync delay, but because the Mapped Key is set to "pressed OR release" means that you should be able to hold the button down (and not let go) while watching the healer's screen to see if you can catch where it's happening. At a quick glance I don't see anything wrong, but it'd help if you could be more specific as to where/when you're seeing the issue occur.