I'm having the same issue. I was able to narrow it down to using a negative number for a series pointInterval. When I changed it to a positive number the tooltips worked as expected, but upon setting the pointInterval back to a negative number, the tooltips again broke. My graph uses a datetime type for the x axis and pulls data from a table. The table is sorted by date descending, so the most recent date is at the top, and I need the graph to have the largest date at the right side and oldest date on the left. In previous versions of HighCharts this worked fine with type of datetime, reversed false, and pointInterval set to a negative number.
Fusher - The link you provided looks like it's still using 2.1.9.