The 1.21 Change Log has the following update in it:

https://www.minecraft.net/en-us/article/minecraft-java-edition-1-21

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ENTITY DATA

PROJECTILES

• Arrow-like projectile data now contains a weapon field containing an Item Stack representing the weapon the projectile was fired from
• The ShotFromCrossbow field has been removed
• Removed power NBT tag and replaced it with acceleration_power tag with a number value that control the projectiles acceleration (and maximum speed)
  • This change affects the following projectiles; dragon_fireball, large_fireball, small_fireball, fireball, breeze_wind_charge, wind_charge, wither_skull

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There seems to be an unintended change now to how much water affects the trajectory of wither skulls. Please see the attached picture showing a big change in the height gain being affected.