The bug

Sculk sensors do not detect when an ender dragon shoots a dragon fireball, despite this being a projectile fired. The action of a projectile firing should count as vibrations, according to the 20w49a patch notes:

Relates to MC-207473.

How to reproduce

The reproduction steps may seem a bit long, but that's just to make it easier to reproduce (it takes a really long time without using these steps). Dragons are quite difficult to work with! I've broken the reproduction steps into four parts.

Part 1 - Preparing the position of the dragon

1. Enter the end (and respawn the ender dragon if there isn't one already)
2. Place a repeating command block near the portal, and set it to Always Active 
3. Configure the command block with the following command:

   tp @e[type=minecraft:ender_dragon] ~ ~20 ~

Part 2 - Preparing the sculk sensors

1. Turn on hitboxes by pressing your F3 key along with B (this is not necessary, but it makes everything a bit easier!)
2. Run the following command just below the dragon's head:

   /setblock ~ ~ ~ minecraft:sculk_sensor[sculk_sensor_phase=inactive]

1. Place a downwards-facing piston under the sculk sensor, with a movable block below the piston (this will tell you whether or not the sensor was activated at all)

Part 3 - Preparing the end crystals

1. Place an end crystal at the top of the end portal, and make sure the dragon connects a beam to the crystal
2. Travel about 20 blocks away in the direction that the dragon's head is facing, and run the following command:

   /summon minecraft:end_crystal

Part 4 - Testing

1. Set your game mode to survival, and run the command:

   /effect give @s minecraft:resistance 1000000 255 true 

1. Make sure the ender dragon is still connected to the end crystal at the portal, and not the one summoned by the command
2. Break the command-summoned end crystal (this will set the DragonPhase tag of the dragon to 1, which is needed for reproduction)
3. Wait until the dragon shoots at you
   Notice how the blocks that would have been pushed by the piston below the sculk sensor remain in the same place as they were in the beginning, meaning that the sculk sensors were not activated