So, almost a year now I've been experiencing this bug, and I still don't have a satisfactory solution.
The only fix that I've found to be moderately successful is by using MCCC and flagging sims 'freeze appearance' and 'freeze physique'. However, this doesn't ALWAYS work (especially if you wait any amount of time to do it after a sim has aged up), plus it sucks having to do it for more than a couple of sims at a time (let alone a town or world), and it prevents sims from getting fat/thin or muscular/weak (thereby leaving no consequences for diet or exercise).
