I'm not sure if it has to do with it being tin, but I had 3 ingots on an anvil, selected axe, and got a single axe blade with one ingot. All three were firmly placed on and it turned two ingots into a single axe blade and left me one. 

Hey Awesomeal15,

Usually putting things on the anvil works well but rarely you'll get an issue with the anvil not registering the ingots. I'll look into adding a feature that will let players know if the game registered 1,2 or 3 ingots. So if that happens, you just need to take them off and place them back on again to make sure everything is registering.

Thanx for the bug report.

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? :)