> Why would they want to?

I don't know, when the board changes you look for openings. On the military side, I would use the distraction to extract battlefield gains. On the diplomatic side, maybe Putin is willing to give up more, now, than he (or his successor) will later.

And Ukraine has the convenient position of just telling Wagner forces something along the lines of "you're a bunch of war criminals, but anyone observed retreating into Russia isn't going to be pursued" if they want to let the infighting do the maximum amount of damage.

Ukraine struggles with ideas what to do with unloyal population on ukranian occupied territories. Main ideas are "we just mass deport locals to Russia" (FTR that's a genocide and not because of destination) and "we just forbid russian propaganda there and everyone would magically start love us and accept ukrainization". And you are talking about russian territories that combined have size of half of Ukraine and which population is extremely unloyal to ukranian government.