It is as the title says. Here's a screenshot:

I don't know if it's because the item that caused this can smelt 4,608 items, but I think it is. As of typing this, the buffer has gone down to -67%.
Edit: The buffer starts at -55%

What fuel you are using?

The problem here is that Aeternalis Fuel Block has a very high fuel value that when get multiplied the number pass the integer max value returning a negative value.

I am using Aeternalis Fuel Block, from ProjectE

(sorry for late reply)

And also, at around -72%, it flips back to positive numbers, so then 72% 71% ect

Ok, that's what I thought was happening. But the fuel still works, it just display wrong