Helmets do not reduce the damage taken from falling anvils or stalactites by 1/4. They still reduce the damage by a small amount, but not more than wearing any other armor piece, despite still taking twice the durability damage as other armor pieces.

Steps to Reproduce:

• Setblock an anvil or summon a falling block above you.
  • This may be easier to visualize if you spawn the anvil higher to deal more damage and disable the "natural regeneration" gamerule.
• Observe the damage taken.
• Restore yourself to full health, and repeat the previous steps while wearing only a helmet.
• Repeat the test again but with a different single armor piece instead of the helmet.

The helmet only provides a small damage reduction in line with what would be normally expected from armor pieces; chestplaces and leggings provide more damage reduction due to having more defense points. However, the helmet still takes increased durability damage.