The tooltip only appears when you hover over the voting icon a second time after voting with the voting power level

Steps to repeat:
1 Click upvote icon
2 Select level of the voting power
3 Click Upvote on the slider
4 Wait till upvote icon change color
5 Hover this icon first time - the tooltip Undo your upvote <some value>% should be shown but isn't
6 Hover the same icon second time - Now you can see the above tooltip