After days of toil, I have found what was causing the borkiness of doom with Moonbeam aging up. A set of eyebrows (Anva's curvy brows to be specific, though I have no idea whether they are just conflicting with something in my game or are broken in general.) I never would have worked it out if it hadn't been for my finding that two of my other sims were experiencing the same issue.
Unfortunately, all three of the sims experiencing this issue are ones I have already uploaded (Murphy's law, much?) If you have downloaded:
Moonbeam Frost
Maisie McDermott
Vada Fennell (either teen or child version)
Please take these sims to a mirror and give them new eyebrows immediately. I'd like to point out that Moonbeam was broken even though when I took her to a mirror it showed her as not wearing eyebrows. Give them a new set of eyebrows before aging them up, please. This will definitely fix any issues.
I have already re-extracted new versions of them with wonderfully non-borky eyebrows by Pixelcat. Any new downloaders of these sims will not experience this issue. And obviously, I will never use Anva's curvy brows on any of my sims again, and will check for the issue before uploading in future.
I'm sorry for any inconvenience, borkiness or heartache I may have caused in your game.
Slightly related to this, I'm still searching for a Frost Legacy heir. I know Moonbeam's not broken, but the poll still stands. It will now be open indefinitely until there have been enough votes to break a tie/until I play the family again.
Please go vote in the poll. It's literally only a couple of seconds of your time, and due to my own
OCD tendencies of doom appended rules for playing an ISBI, I'm not allowed to choose one myself in the event of a tie.