Mobs don't burn in tall grass, large ferns, peony, rose bushes, lilacs, or sunflowers. Can't tell if intentional or not, it feels weird that majority of their body is showing, but somehow don't burn. (Especially the sunflower one)
Mobs run to hide in the double block high plants during daylight which makes me think it's intentional, but I can't find anything on google or minecraft wiki about this.
Mobs also don't burn when you push a solid block into their lower half with a piston.
P.S. it looks funny when you're in a plains biome and there are skeletons everywhere hiding inside long grass blocks.
MC-123853 Mobs don't burn when standing in double plants/cobweb