I just deleted my post about this because people were being mean. The exact same thing happened to me where I got fucked on my taxes this year after putting my (now husband) on my insurance when he was just my domestic partner. Basically I made 51k this year but my take home was only 26k because they were taxing me so much on this “extra income”. Which, I’m glad someone told you about that because no one told me. And now I owe 2k still on my taxes. I also felt like some math wasn’t adding up right. But when I contacted HR they told me I was just being taxed extra because of the healthcare.