Ideas
Ah, right! Up to your old tricks, Electronic Arts? I'm having the same bug that you fixed back in version 1.48.90! At least ChatGPT found confirmation: "Cat Whiskers now appear with the proper color relevant to the color chosen in Create-a-Sim. Fixed in Patch 1.48.90." I thought the issue was with the crescent moon mod, but no. Other mods for Create-a-Sim only affect characters. But the point is, the mustache can stay the same length, or it can get shorter. The color, however, is glitchy.
UPDATE: I figured out that the black color is buggy for me. I'm trying to edit a pet in CaS and see white whiskers. I select black, which is clear from the icon, but the whiskers are white. I change it to another color and then back to black—the whiskers are black.