Yeah, they’re both thoroughly evil slime with pretty much identical, hateful politics. So it makes sense they gravitated towards each other. It wasn’t a short fling either—they dated for over 10 years.
I’d guess what eventually broke them up is that Pierre needed a younger, photogenic tradwife to have some kids with, as part of his image make-over in pursuit of becoming PM.
Hell, I wouldn’t even be surprised if it wasn’t a case of one or the other being dumped, so much as the two of them coldly planning it out together and deciding it was their best path to getting into power and enacting their shared political agenda.