So I've discovered two blocks that create burnable blocks when used on carpenters blocks when the base material is not:

Seared bricks from Tinkers Construct
Sheet Metal from Immersive Engineering (not the iron variant though)

I saw #204 though, so is this something to raise here or with the other authors?

If a block uses a tile entity it will internally be represented as a wood block. That is probably why they are burning in some cases. That's the only thing I can think of for this scenario.