Modifying the font properties (pixelSize, bold, ...) in a tooltip works as expected if a literal is used. As soon a property or expression is used the font family falls back to default.
In a text control using expressions for font property works as expected.
See attached main.qml and screenshot
Used font is [Fira Code|https://github.com/tonsky/FiraCode,] but any other family will have the same result