Hi Michele,
I was able to reproduce this issue in Domino 9.0 on that sample that was working in 853. Though looking at the console output from the browser I did not see any error - just that the tooltip no longer displayed.
I also checked this issue on coming Domino release, 9.0.1, and here I've not been able to reproduce this issue there. So it looks like that this issue has been indirectly fixed.
I hope this helps,
Paul.