Thanks for that information Dimitar. That's the behaviour I'm observing as well. When I click on another element or outside of the dropdown it re-validates and clears - although that's extra work for the user.
Also the proposed solution is great when there is only one error showing however that isn't always the case. It seems to me that hiding by class will hide everything, and tooltips don't have an ID associated with the control. (Although I could perhaps position them in a span as I've seen before) - in this case extra work for me.
In the select event, would it be possible to trigger a blur and force the focus to the next element in the tab order? Would that hide the tooltip automatically? As a general rule I'd like to auto advance the focus when possible so this could help in other ways also.