I dont think that's a bug exactly. You lose 15% of xp for the dark forge per item. 10 items is just 150% or negative 50% xp
Im with Kami the Text is:
„When the carrier gains experience, 15% of it is lost as a Tribute to the Forge“
So it shouldnt touch already gotten exp. I already told Andy about he was like:
„Well then, make Iron cant merge those Items“
In my opinion that isnt the right way and it shouldnt go negative as well...
I can see that side of it. But also knowing that the one dark sword that's like excalibur does actually drain xp every few seconds so negative xp item does exist. I think that if you were to cap xp loss at 100% it would still be a bit over powered being able to fuse 40+ dark items without any serious consequences. But that's coming from someone who never uses iron man.
I guess I just always saw dark items as a double edged sword and accepted the way it was and laughed when my iron man kept down leveling
I think the way to go is pick Osmo hero, get the -25% bonus xp from that, then combine it with Useless machine, that gives -80% bonus xp. Together, you have -105% bonus xp.
Since the item works by subtracting xp each time you gain xp, this should block the dark item negative effect.
Example:
Currently you wear a few dark items and it looks like this:
Gain 1 xp. Each dark item then says, ok, I will steal 15% of that. So they take 0.15 per dark item. 7 or more and you are losing xp overall.
However, if you have -105% bonus xp gain, then the net xp is -5% xp. This means every time you would try to get xp, the game will probably give you 0 xp, and the dark items will take 15% of 0 xp, which is also 0.
There's a small chance the gain will let you lose xp with useless machine and Osmo, but I doubt it. If the game does let you lose xp, then stack a bunch of dark items to give your Mr. Iron lots of xp.
However, remember that all xp he gains from dark items would be stolen from Dark Forge.
A statement of Andi is found here
https://mazebert.com/forum/bugs/delevel-wanted--id1281/