It feels like another case of software trying to be helpful … and I think I'd prefer to turn it off. Is this possible?
Unfortunately not. FM uses an automated algorithm to adjust line spacing parameters based upon the font size used. You'll have to manually change those in the designer after the font size is changed.
Changing the paragraph leading based on the font size could generally be helpful, even if I tend to find the defaults a bit cramped. Changing space before is just meddling