Recently I've been having issues with my sketched lines not constraining. For example, if I were to do a sketch on the face of a square model, and I 'Convert Entity' the outside profile of the face so that I can sketch a line from a midpoint, the sketched line doesn't constrain itself at  the specified midpoint. I can grab the line (Or the endpoint) and drag it where ever.

This is a recent issue for me, so I'm assuming I accidentally unchecked something in the settings, but I'll be damned if I can find it.