I don’t think this is intended. I managed to make two adamantine items, though I didn’t have two lumps of ore on me during the golem fight, so from memory my sequence after it was (1) retrieve my adamantine armour, (2) raise the platform, (3) collect another lump of ore, (4) eject the old mould, (5) insert a new one, (6) insert new lump of ore, (7) use the hammer lever to go back to the lower level, (8) open the lava flow, (9) use the hammer lever to make my new item.
But if you have a second lump of ore, I don’t see why you’d need to go up and down again. As long as you have new ore in the crucible and a mould in the appropriate place, and the platform is at the bottom, I’d have thought you’d be able to operate the lava flow.
If saving and reloading doesn’t help, then it sounds worth sending your savegame to Larian via their support website or the feedback form on the launcher so they can investigate.
