When the heat screen appears at the end of a depth, it often shows my items heat filling a little way, say to the halfway mark. But then at the next depth it says that my items are fully heated and can now be upgraded. Why is this? Is it a graphics bug or am I just an idiot (don't answer that).
Quick Question
I most definitely didn't
This has happened many times to me... Also there is no way a 25% heat bonus could fill a heat bar up twice of what it originally displayed.
This has actually happened to me quite a few times as well since the Big Update. I don't think it's a graphics bug, I think it's probably something that hasn't been transitioned correctly into the new system. But what do I know.
Sprites.
The sprites visually halve the heat shown on weapons/armor for that menu.
If you dont have a sprite that can take heat (waiting to lvlup or none equipped), you wont see this bug happen.
It is also affected by items that are currently maxed out at a level, but have not been forged up.
Also, as Skepticraven said, Sprites affect it the same way.
Also, if you have other gear equipped that was previously ready to be forged, these weapons also get the level heat "applied" to them in the menu, further distorting the issue.
Edit: Sniped :P
It's not a bug. It's likely that you got a heat bonus when you forged your item. Did you remember getting a heat bonus?