I've got a scenario where a tooltip is not hiding when another tooltip is shown, and here is a dojo for reference.
If you move your mouse up and down the colors, the tooltips show and hide as they should. If you stop on a color, move the mouse right into the tooltip, then straight back out of the tooltip and back into the color for that tooltip, then move the mouse up or down over another color, the first tooltip doesn't hide. You can repeat this sequence for the other tooltips as well, resulting in multiple tooltips showing.
Here is a jam so you can see what I mean as well. You can't see the mouse move, but you can see the multiple tooltips.