See summary.

What I expected to happen:

The dragon would take damage like every other mob/entity that takes damage

What actually happened:

No damage was dealt to the Ender Dragon

How to reproduce:

1. Place a block of sand and put a 4-block tall stack of cacti on it
2. Set gamerule mobGriefing to false
3. Stand next to the cactus and do "/summon ender_dragon ~ ~ ~ {DragonPhase:0}"

          →  The dragon doesn't take damage from touching the cacti, which can normally damage even the wither.

This bug probably occurs because dragons don't have collision with blocks. This also contradicts the following statement:

"When any entity that takes damage, including players and mobs, touches a cactus, 1 damage is taken every 0.5 seconds."