Hey!
This is something that's been in STM for a long time, and I kept wondering if I should change it or not... essentially, it works like this so that as it inserts hyphens, the rest of the mesh doesn't change to accommodate this, and manual hyphens get included in this too.
I really should revisit this and make it work in the way that would feel more natural, though. I feel like I would have tried to make it work that way and hit some wall in the past, but maybe I can come up with a real solution, now.