If you look at the top half of a double high rose flower, the model is "flower_grass_bottom". The bottom half says "flower_rose_bottom". Also happens with peonies. Peonies say "flower_fern_bottom".

What I expected to happen was:
The top half would say "flower_rose top".

What actually happened was:
It said "flower_grass_bottom"

Mod edit:
Lilac is ok, Sunflower calls the top block "sunflower_sunflower_top" instead of "flower_sunflower_top"