Well, I guess they would have to look to the agreement that was arrived at to see if it made an exception for circumstances like this, or for Debaltsevo particularly.
if it dose, then the Ukrainians need to abide by it according to whatever terms are in place that were agreed to.
If it does not, then the separatists need to let them move their heavy weapons back per the agreement.
With such a situation going on while they negotiated, I cannot believe that such an agreement would have ignored that situation. My guess is (without knowing specifically), in the absence of anything to the contrary, that that area was treated like the entire front and the agreement covers it as written.