Lava with a block directly above it doesn't spread fire to flammable blocks

I don’t know if this is intended, but when I looked it up, google says that a lava source completely surrounded by non flammables will prevent fire spread. However, if one is to simply put a singular block above the lava, in a sense shading the lava, then even when the lava is directly touching an adjacent trapdoor, no fire is spread/produced. Again, I don’t know if this is intended, although I don’t believe it is.