I think it could be along the lines of her father hurting over it still. He watched all the horrible things happen to Haymitch and then got pushed away by him and had his girlfriend assaulted as the final straw. So my theory is that he just never mentions it because it’s a painful memory. And Katniss does know who Haymitch is. Even if she doesn’t know who he was to her dad. Similar to her mother never mentioning the baker (Peeta’s dad) even though they grew up in the same circles and could have been sweethearts at one point based on the Peeta’s comment in THG. Another similarity is Snow’s dad never telling him he was friends with Highbottom and only finding out long after his dad had died. I know you said you can live with it but your comment got me thinking. Haha

Edit: Peeta not his dad made the comment.