I think I tested a similar sized bit - perhaps PIRs are not all equal? Mine charred but would not light, but as I mentioned, did produce some nasty smoke.
It's a little worrying as you really don't want that lot going up in a roof!
I am covering 95% of mine with plasterboard (essenstially down to within a foot of the eaves, not practical to got the last bit), even in the attic spaces, so that will reduce the risk to very small... (I hope!).
Oddly the BCO mentioned nothing either way and he knows I am using lots of PIR.