Well, if you are using the ER70S-2 or E11018-D2 and the weld comes into contact with the overlay (i.e. gets diluted), then hot cracking as you describe is not uncommon. If you have to weld the joint in contact with the overlay, use nickel based filler like alloy 625, either that, or strip back the overlay so that the carbon steel filler doesn't come into contact with it.