Wrong magma solidification

Im using volcano like energy generator. Aftger magma cool down to igneous rock, robominer makes space for fresh magma.

After few cycles, im here:

Upper row have mass over 20 000kgs, but normal lava title can have only about 1800kg.

And game reload after two cycles:

Mined igneous rock (with temperature under melting point) was changed to new row of material - this causes damage of my robot miner & his cooling

Game build: AP 420700-D