[V] vampire sims born in game have darker skin tone for 'dark' form

[SerenaBeanie]

Product: The Sims 4
Platform:PC
Which language are you playing the game in? English
How often does the bug occur? Occasionally (10% - 49%)
What is your current game version number? 1.90.375.1020
What expansions, game packs, and stuff packs do you have installed? high school years, cottage living, snowy escape, eco lifestyle, discover university, island living, get famous, seasons, cats & dogs, city living, get together, get to work, werewolves, my wedding stories, dream home decorator, realm of magic, strangerville, jungle adventure, parenthood, vampires, dine out, paranormal, nifty knitting, tiny living, moscino, laundry day, toddler, vintage glamour, romantic garden, movie hangout, spooky, perfect patio, holiday, decor to the max, incheon, fashion street, courtyard oasis, country kitchen
Steps: How can we find the bug ourselves? set up neighbourhood stories so that vampire sims will reproduce. play awhile. check in on born vampire offspring once they age up to teens (and gain a dark form)
What happens when the bug occurs? the dark form does not resemble the human form of the vampire sims, and often has a darker skin tone in comparison
What do you expect to see? the dark form and human form should at least resemble one another in terms of genetics, with differences like skin details. especially so if the vampire parent(s) dark forms and human forms were nearly the same.
Have you installed any customization with the game, e.g. Custom Content or Mods? Not now. I've removed them.
Did this issue appear after a specific patch or change you made to your system? No

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there haven't been mods in this save file since before these sims were born.

unsure if it's relevant, but the offspring shown were the result of a vampire & werewolf pairing. they show up as vampires (non hybrids) in CAS. I've seen this bug happen before werewolves was released as well. I haven't made any edits to the sims to have them show up this way. the vampire parent's dark form (attached screenshot) very closely resembles her human form except for skin details.