我目前还没有足够的代表来发表自己的答案,但这是在浪费更多人的时间之前：

Ok, I now understand why I don’t get all of the expected ancestors:

JQuery datepicker deletes the parent node (the datepicker-head)
onclick. After that my event is triggered on the button of the
element that already got deleted. So now I am “trapped” in the scope
of the deleted element and can only traverse up to the deleted
container itself. I think I can work out a solution for my
event-delegation with this knowledge!

I will add this as the proper answer to my problem in 8 hours!