Oops! I probably should have added the information about the lipring on the main post! Let me just copy and paste what was discovered about that:

"I opened up the sim from my previous "piercing28," who was made when I used Rick & Delphy's slider hack, and fidgeted with his lower lip sliders. Lower Lip Shape was maxed (to the right); as soon as a decreased it, the stretching reverted into a neat little circle. I re-maxed the same slider, and it the piercing remained unwarped;and the sim's mouth-shape was different (a lot less full) than the original Rick & Delphy slider hack results. (However, increasing the Lower Lip Thickness created a near-identical version of his previous appearance, without the distorted lower lip ring.)"

(Gobs of love to Juushika on GoS for figuring all of that out!)

Because of the way facial slider hacks seem to work, coupled with how I've done the bone assignments on the lip piercing so it moves with facial anims, this is something that's just going to have to have a warning slapped up on it. XD

So now I need to ask what facial slider hacks you're using, and what they're set to. So far I think you're only the second person to come across this! Thank you so much for reporting it and getting me the screencaps!