Okay, I restarted the game about an hour ago, and just now advanced beyond the Cup Bug. To be on the safe side, if you already filled a cup with Molten Glass, and used it to acquire the Leg Lamp, do NOT fill a second cup with Molten Glass. This time around, I used Hot Grease on the Leg Lamp, and the cup remained.
In any case, even if you encounter this bug, you will be fine so long as you remember to acquire the Hot Wax and Mysterious Caustic Fluid in one of the remaining cups. These two ingredients (next to Volcano Shot, which is always in your inventory) are all that you really need to complete the puzzle, anyway.
Telltale, here is my old save file, if it helps: