I put 3 ingots firmly on the anvil, just like the other millions of times I'd done it. I chose the axe and I got a single axe blade that took 2 ingots and left one behind.
Should probably take a screenshot next time it happens (use Windowed mode with your screen resolution set at the in-game resolution, It'll look fullscreen and act it for the most part (then hit prt sq/prtscn key, paste it to mspaint if you got nothing else))
I have to assume something blocked the activity if all three ingots popped into place. - A Handle, other item, shield, frame, crate, anything, if anything is touching the anvil, Make sure it isn't nearby - You only want the ingots you're working to be near the top of the anvil, and preferably, you want to only have ingots near the anvil unless dismantling. Anything can cause an ingot to fail to react even if its just barely near it, since there's some hitboxes to some items that are fairly large considering their purpose (Connecting trigger boxes - for connecting items together/activating combination)