Put the summary of the bug you're having here

What I expected to happen was that the arrows would bounce off the Ender dragon as the type of arrows they were originally. Sometimes, they also dealt the damage afterwards, once the dragon took off.
What actually happened was that they changed into fire arrows after hitting the Ender dragon.
How to do this:
1. Get to the End.
2. Destroy the crystals (optional).
3. Wait until the dragon sits upon the structure of bedrock meant to take you back to the Overworld.
4. Take a bow and shoot the dragon; you will see the arrows catch fire.