I made a little column of diamond window tiles to tap magma. There was a bit of iron and abyssalite that dropped in the magma but otherwise nothing fell in there. This drop of molten iron turned into a pile of debris after it cooled, and then over 13 tiles of magma also turned into igneous rock debris and created a vacuum pocket where the molten iron was. It should not be dropping as debris and should be making solid tile with an entombed iron debris. Save file included
I have recreated this in sandbox like so:
Spawn 200kg of molten iron surrounded by 1840 kg of molten magma on either side. Then make some very cold super coolant on top. When the iron cools first, it will turn into a debris pile. Then the magma on the right tile turned into debris instead of a solid tile like it should have.