It's a case not properly accounted for yet, so the general purpose code is running instead of proper handling for super high mass bodies. Which would not allow any mass to escape.
I'll try to get that fixed as soon as I can.
"Right now all of the impact melt is coming from the body being hit. So they can lose rather large amounts of mass to fragmentation, depending on the impact velocity.
I'm working on having the mass come from both the impacting and impacted body instead. That should fix a lot of these odd cases with loss of mass. Until then, there's going to be some weirdness."