The Bug:

Sculk sensors are not activated upon fire or lava burning items.

Steps to Reproduce:

• Throw an item into fire or lava.
  →  Notice how when the item burns, the sculk sensor does not activate.

Additional Information:

Please note that the item must not hit the ground in order to reproduce this issue. Quote from the 20w49a Changelog, "A vibration is anything that causes physical motion". I would definitely consider this as physical motion, as an item is physically burning.