Okay, so I prepped, got to the Adamantine Forge, put the necessary items in, hammer strike lowers the platform. Use the lava valve, fight and beat the boss, grab my item. Put in the necessary items for the second mithral ore, turn on the lava, use the hammer. Lava's still there. Use mould ejection, get the second item, try to use the platform button, and... nothing. Try to use the lava valve again to possible turn off the lava, nothing.

Is this a bug or am I missing something here?

Actually, nevermind I guess. I quicksaved and quit the game. Reload in, the lava went away and I could use the platform button. Really weird.

yes the lava sometimes get stuck a bit

out of interest, what was your 3 items?

The lava getting stuck is an Early Access bug which happens if the player releases the lava during real-time mode. So to make it go away simply enter and exit turn-based mode.