I am not really sure if that's a bug?
I had the same issue but after locking at the third ingot, i noticed it had cooled down and wasn't heated anymore.
I don't know how you placed your ingots, but i have multiple ingots on heat all the time.
So i think i just took 2 well heated ones and one ingot which was already cooling down.
Maybe it was the same misstake for you? :)