Use scenebuilder and add a label. Change the font size of the label to something arbitrary (e.g. 48). Drag a tooltip and drop it onto the label. Set the tooltip text but leave the tooltip font on the default settings (System - 12px).

Run the app. The tooltip font size will now be the same as the label font size.

Note, if you change the tooltip font size to anything other than the default (i.e. 12), then the tooltip font size will be correct. If you change the tooltip font size to 12 (the default value), the tooltip will be displayed in the label font size.

I also noticed another problem when testing this. If you change the tooltip font size (to anything other than the default 12), then every time you mouse over the label, the tooltip font size shrinks and the tooltip gets smaller. That seems like it's another issue.

Note, my tooltip is set as an internationalized string... just in case it doesn't reproduce as a standard string.