Is it not the case that MCP ALT controls in TOGA & LVL CHG and FMC ALT controls in VNAV?
Yes.
But only if the MCP ALT is
above the point where you engage TOGA. If you forget to set the GA ALT in the MCP (having maybe set a lower MCP ALT to avoid ALT ACQ) it will climb till it runs out of petrol.
If at some point you realise and hit LVL CHG (especially if the MCP speed is high) it will hurry towards the ground.