Not sure why you’ve never
Not sure why you’ve never noticed it before – are you using the same kind of shelf paper? Are you typically in the same room when the kiln (with shelf paper) is between 300-ish and 900-ish degrees F?
If you haven’t moved any of the glass – go ahead and refire.
The reason for the discoloring on the shelf this time is that you stopped the firing before all the carbon burned away completely. Unless I’m compltely missing something here, the shelf should return to its normal color the next time you get to 1000F.
Helios Kiln Glass Studio