This is quite hard to describe so bear with me. When I front squat, in the very bottom position it feels a little like a sort of lose tightness and collapse forward ever so slightly. I don't lose tightness in my upper or lower back (as far as I can tell - my lower back usually screams at me if I lose tightness), but there's definitely a loss of tightness somewhere. It causes discomfort and sometimes pain in the upper, outer region of my calves if that helps at all. My knees are not collapsing inwards when this happens (again, as far as I can tell).
It's only the very last few degrees where it happens. If I stop just short of rock bottom then I keep the tightness all over, but ideally I'd like to address the issue so I can keep squatting all the way down.
Thanks for any help you can give, and I'm sorry my description isn't very good.