What Sandwich said.

Basically, the game thinks it should be giving heat to all your gear that's not at max Level. In this case, it gives heat to items that are at max heat and need to be forged. So it visually spreads out all the heat as it thinks it should be doing, but as Sandwich said, in reality the heat is still given as it should be given (that being it gives all the heat to equipment that hasn't reached full heat yet).

You can start to kind of predict whether or not something will get max level by looking at how much gear is getting heat. You just need to get used to it. Otherwise just check the forge every single depth to see if you can heat something yet.

The visual bug is because of battle sprite "heat", without, the display should show exactly whatever the heat at.

unequip your battle sprite? <__>

This only happens when heating 3* and below
Max heat battle sprite (at any level ) doesn't eat up any heat
but max heat gear ( below lv 10 ) does

I'll just make up a quick example:

Okay, say all your stuff except for two items is at Level 10 (let's call these two items Weapon A and Shield B).

You're currently heating Weapon A.

Shield B is currently at Level 7. It has maximum heat already, but you don't want to waste your crystals to forge it.

At the end of the level, you begin getting your heat for Weapon A and Shield B (since everything else is maxed out, these would be the only two items being heated).

Here's where things get trippy.

The game still [visually] thinks it should be giving heat to Shield B, even though it's already at maximum heat and needs to be forged.

So instead of Weapon A getting all the heat, it only [visually] gets half the heat (due to heat being spread out evenly among all equipment).

However, in reality Weapon A is still getting all the heat, it just visually looks as if it's only getting half the normal amount because, visually, Shield B is taking half the heat.

When you get to the next depth, in reality, Weapon A has actually received the proper amount of heat, and Shield B has not taken anything away.

It's just a visual glitch that they have yet to patch.

Now, this is just an example. In-game, you'll have many more items. Visually, the heat gets spread to anything not at Level 10, but in reality, the heat only goes to those items that don't need to be forged yet.

And THAT is why you can "forge something that isn't fully heated". Because visually everything is being spread out, but in reality everything is still getting its proper heat.

Don't believe me? Take off your Sprite and use only Level 10 gear except for one item. When you finish your depth, the item will get its heat. When you look at it on the next depth, you'll see that the item got the same amount of heat it looked like it just got. Why? Because the game has nothing else to spread the heat to visually.

Hope that clears things up.