Just wanted to chime in and say that I’ve had this exact issue with one of my sims. I believe I got lucky and managed to do away with the fear of the dark bug by chance. Initially my sim developed fear of the dark, then had the “become confident outside at night” want, and when I fulfilled that, he “overcame his fear”, but the fear of the dark moodlet persisted outside during daylight hours and at night. Ultimately my sim happened to re-develop the fear of the dark trait by chance again, and then I purchased a fear-be-gone potion with aspiration points and used it to get rid of his fear of the dark. By some miracle, that worked, and the bug disappeared altogether.
This is of course entirely anecdotal and may not work for anyone else, but I wonder if you’re stuck with a sim with the fear of the dark moodlet with no fear of the dark trait if you might use cheats to give your sim the fear of the dark trait again, then try to cure it with the fear-be-gone potion? I haven‘t tried it using cheats myself, since as I said, I got pretty lucky, but I feel like it’s worth a shot. Just wanted to give my two cents, since I had to deal with that bug for quite awhile, and it was really obnoxious. Good luck!
EDIT: I just read back about how some people have already tried adding the fear via cheats and tried curing it with the potions; my friend also reported having the same problem even after attempting that solution. Evidently her sim also re-developed fear of the dark naturally w/o cheats, and even then using a potion didn’t fix the bug, so I’m honestly not sure why it worked for me ☹️ On the off chance that I happen to find anything new, I’ll keep y’all updated.
EDIT AGAIN: My friend ultimately just installed a mod to remove individual sim moodlets in order to deal with the bug. It isn’t ideal, but it at least works, so if anyone else is interested in using the mod in the meantime, until the bug gets patched, here’s a link: https://www.patreon.com/posts/ui-cheats-v1-16-26240068
