This is a bug in the minecraft 1.9/snapshot version, I couldn't select the correct version.

When you place a block at the bottom-half of double tallgrass, the top changes into a flower for a tick or so and then dissappears. It only happens if you place a block at the bottom-half, if you punch the bottom-half it doesn't happen.

Also, I've managed to get 2 different kinds of flowers on different worlds:

• The top half of a sunflower on my private worlds.
• The top half of a peony on my friend's server (has nothing to do with the sugarcane I used in the picture).

This seems to be very similar to a bug that used to happen when you punched to bottom half of the Double Tallgrass and the top changed into a flower for a moment.