The portable forge you deploy on the ground in front of you only lasts about 8 minutes 20 seconds.
Also there is a bug that causes it to remain in your view even after it poofs, until you zone or log. If you have the forge window open and try a combine but get the message "Your request failed because the item was not found." that means the forge poofed on you while you were using it. Or if you click on the forge and nothing opens except your inventory window, that also means the forge is a phantom and not actually there anymore. You'll have to deploy another forge.