Just noticed that if you do have a field name/ measure that contains a colon ( : ) that has spaces either side it will not display the value on a tooltip, it will be blank. If your field name end in colon it will not appear on the tooltip at all.
It is unlikely that you will usea colon but I had just the experience now and have spent 30 minutes trying to work out why!
"Test" and "Test : A" are both exactly the same measure but the field name with a colon will not display as a tooltip.
If the field name ends in a colon the tooltip pane is totally blank.
I have reported this issue internally and post back once I get updates.