The Bug:

minecraft.used:minecraft.lava_bucket doesn't increase when filling a cauldron with a lava bucket.

Steps to Reproduce:

• Create a scoreboard objective for tracking the use of a lava bucket and set it to display on the sidebar.

/scoreboard objectives add UseLavaBucket minecraft.used:minecraft.lava_bucket
/scoreboard objectives setdisplay sidebar UseLavaBucket

• Fill a cauldron with some lava using a lava bucket.
  →  Notice how the scoreboard doesn't increase.