Looks like it works on small text since both tips are perfectly superposed.
Have to test on a long description
I think this is since tooltips are automatics on grid.
The automatism is supposed to detect manuel tips, but looks like it does not works for real
A solution can be to simply remove the "_renderWithTooltip" in CacheTool if basic tips are ok.
Another way to fix is to find why the automatic tips is not deactivated...
Or both maybe is the base
If you move your mouse a little between the first tooltip and the second tooltip apparition, they are not perfectly supperposed. Both tooltips are not displayed at the same time.