(This also afectt versions 1.21.60 and 1.21.61)
If you place End Crystals next to the Ender Dragon, it will connect to regenerate. But if the crystal explodes, and the ender dragon is connected, it will take damage. The problem is that, from 1.21.60 until now, crystals naturally generated in towers or those placed by the player (both examples in the video attached below) do not take health from the ender dragon.
Expected result: The Ender dragon should lose health, as it is supposed to work.
Actual result: The dragon does not take damage if it is connected to a crystal.
How to reproduce:
Go to the End. Search or build a portal.
Pick an end crystal, and wait for the ender dragon to come closer.
(NOTE: Don't blow up the crystal too close to the ender dragon, in my tests I found that the dragon doesn't take damage from the lightning as it should. Instead, if it gets close enough, it will take damage from the crystal explosion. So, you have to blow it up from a certain distance.)
When he gets close, explode the crystal and check his health bar.
If you want, you can also check it by placing a End Crystal on the ground and repeating step 4.
It's because the crystals heal the ender dragon so it's hard to do it you need to explode the endn crystals before fighting or explode it on him of there's one left