This was previously posted as a feature request (there are screenshots in the comments and a link to a general complaint about the font size UI):
I don’t see this behavior anymore — the tooltip now appears above the slider. But it can still end up under my finger because the interface itself changes size as you move the slider, which is disorienting.
If possible I think it would be better to use a preview box to show the size change, and only apply the change everywhere when the slider is released. Also to show the font size in a permanent box next to the slider: