Just submitted a report after experimenting with this a bit, but wanted to see if anyone else had encountered this problem/bug as well…
Customized tooltips in the chart element will not display at all. Tooltips will only display when they are not customized and, as a result, number formatting cannot be applied (percentages will display only as integers, the number of decimal places cannot be set, etc).
Doesn’t seem to matter whether or not a style is applied to the chart, or the type of formatting option selected.