This happens on Firefox (i`m using Firefox 43.0.4). Tested on Chrome and IE and it works fine.
See the fiddle: https://jsfiddle.net/frozen0k/tsp0grmb/
When the tooltip is big and it appears over the mouse, sometimes the tooltip dissapears. In max. 10 tries, hovering the link, you should see this happening on firefox.
edit: Increased the delay time before tooltip appearing to 2 seconds and the issue became more obvious.
change the position (
Sure i could do that, but that doesn't mean is a fix :p
Until a fix is made, i use the target: 'mouse' and at: 'top right'