Hi @louloucasio,
That's just the way InDesign works -- it allows swashes and suchlike to extend beyond the limits of the containing text frame.
Fit Text follows suit: It will enlarge the contents of a frame until it causes text to become overset, and then it dials it back one notch. So if InDesign allows this, Fit Text will too.
What I would suggest is to add an inset value to your text frames. You'll have to add as much inset as needed in order to keep the swashes within the boundaries of the text frame. There's no formula to calculate this, so you'll have to eyeball it...
