Steps to reproduce

• Find/place a double plant (e.g. double tall grass)
• (DO NOT use a peony for this)
• Replace the bottom part of it with a block
• Observe carefully

You will notice a peony flash on the top part, this is because the top of the plant takes a tiny bit of time to disappear, but for some reason, it becomes a peony rather than the correct type of double plant.
This applies to all double plants, but you can't notice it with peonies.