You can decrease the level of a water or powder snow cauldron, through the use of an entity on fire in spawn protection

The Bug:

You can decrease the level of a water or powder snow cauldron, through the use of a flame bow in spawn protection.

Steps to Reproduce:

• Start a server and join it.
• Give operator permissions to yourself.
• Set the world spawn to your current location.
• Place some powder snow or water inside of a cauldron.
• Obtain a flame bow.
• Remove your operator permissions.
• Give operator permissions to any person other than yourself.
• Shoot an arrow using the flame bow into the water or powder snow cauldron.
  →  Notice how the level of the water or powder snow cauldron is decreased, despite it being in spawn protection.