magma igneous rock bug?

I have seen this as well.

it happened to stop at 7.8 million for no discernible reason or cause

This seems to happen most often where there is a phase change. I've seen it when contaminated water turns to dirt and steam. You get a bouncing dirt that constantly increases in mass.

have the same bouncing and mass increasing when cooling down gases like co2 for storing in solid form. even when no more co2 is pumped into the room and its vac its raises in mass

after next save and reload it changed to a mineable block of co2 f.e. witch can cause gamecrash when mined but more important blocks the sealed liquid gas container i build