@KushKylie, This sounds a lot like what a poster from the link I gave you experienced. (Click on the link above and see pages 2 - 3 regarding the poster Behappy1st). It also sounds like the bug on page 3 that JPkarlsen gave a link to. You can read that here: https://answers.ea.com/t5/Bug-Reports/UNSUPPORTED-Vampire-Alien-hybrids-bugs/td-p/6143808/page/2
The main difference with those cases are that they are either created with a cheat or they are the result of a male Vampire being abducted by Aliens and resulting in the bugged offspring. Since your case was caused by neither situation, you might want to add your experience to the Bug Reports thread that I linked to above.
After reading everything, I am going to stop testing my family for you and I am going to try to get an Alien couple to have babies with the On a Dark Ley Line lot trait like you did, and see what I get! 🤓😉
(FYI, from the Vampire male and Alien female, they gave birth to an Alien girl! She is just a toddler, but she has all the hallmarks of an Alien, and none of the ones of a Vampire. 😉) I'll get back to you soon with the results of my trying to replicate your situation! 😇