There was a proposal to let them leave, but for whatever reason that wasn't agreed upon. So the separatists kept attacking and Ukraine mounted a rescue mission. They should have cleared up what to do about it in the agreement, but they didn't.

Your point was that it was the Russians/separatists fault that Minsk II failed, because of Debaltseve, but I don't think that's fair. The Debaltseve issue, and the fact that it wasn't addressed in the agreement, was, even at the time, in Western media and in Russia, criticized. After Ukraine broke out and Debaltseve was captured, fighting was massively reduced even there, and heavy weaponry was pull backed from the front line, so that wasn't the thing that sank Minsk II.