The bug
Sculk sensors are not activated upon applying a dye to a sign. According to the 20w49a patch notes, any physical motion is considered a vibration. As the player is performing this task, according to the patch notes, this should be considered a vibration.
How to reproduce
Place a sign
Place a sculk sensor next to the sign
Use a dye on the sign
❌ The sculk sensor is not activated
Can confirm.