Ex constantly talking crap to his current girlfriend.

My ex is a inconsistent deadbeat and does honestly nothing for our kids, except maybe a visit every couple of months. He now has a new girlfriend with a child of her own, and he doesn’t want her to find out he’s a deadbeat to his own and since day one has made 101 excuses and blames me for keeping them away, only wanting child support blah blah. Lately tho, he will tell her lies, to which she messages me believing what he says, I tell her it’s not true and I call my ex to see what’s happening just for him to turn around and say he didn’t “tell her that at all” but I know for a fact he talks a lot of shit, he just doesn’t like being called out on it n it pisses me off especially when we both know it isn’t the truth. My question is, why does he do this? Why would he tell lies to his current girlfriend knowing she’ll ask me, just for me to confront him and he says it never happened/was never said? It’s annoying being dragged into their bullshit when all I’m worried about are my kids, but he has his girlfriend of the month thinking I’m hating, when all I know is she’s only gonna be hurt next as soon as she lets her walls down for him.