I had a look at this, and it appears to be tied to some hidden buffs that a child sim gets when they have a loose tooth:
buff_Lose_Teeth_Missing_Tooth_BedSwipe
buff_Lose_Teeth_Missing_Tooth_AppearanceModifier
buff_Lose_Teeth_Fix_Tooth
These buffs apply an appearance modifier (something which alters CAS parts for the duration of the buff), to children (which works fine), but since these buffs don't get removed when they age up to a teen, the game tries to apply the same modifier that is meant for children, but on the teen, which causes the visual issues we see here. Also, these buffs have an accompanying commodity (which is basically a statistic, kind of like motives) with the same name as the respective buff, and from my testing, removing these buffs alone is not enough to clear the glitch, you also have to remove the commodities, which in the vanilla game is not the easiest thing to do (in theory it is possible with testingcheats, but the cheat is quite long).
However, even after removing the buffs and their commodities, the sim may still have an odd appearance (have very big eyes, something that is usually from very broken CC but not in this case) but it is easily fixed by editing them in CAS (full edit mode) and randomizing their face preset - then edit the sim as normal; the bug should not return when you leave CAS, provided all the aforementioned buffs and commodities have been removed. I have also made a workaround modfix that does this, which should prevent the issue happening on any future sims, but it's not retroactive for Sims which already have a bugged appearance - you would still need to edit them in CAS as mentioned above to get them back to normal, or roll back the save to a point before you got the glitch.
