This is a very interesting bug that I discovered recently and it affects the stairs added by both Thaumcraft and Thaumic Augmentation. Basically if any mob that burns in sunlight (such as zombies) are under any of the stairs, they'll burn like there's no block above them. This isn't affected by vanilla stairs or any of the other modded stairs that I tested.

There is only one line of code that's set on the stairs over vanilla, so something tells me that might be responsible...

So after looking into this more, it seems like that might have been a side effect of allowing impetus beams to go through stairs. I think not allowing that is fine, but not in a patch update since it might break people's contraptions. This will have to wait for a major (breaking) release, sadly.