The Bug:

minecraft.used:minecraft.water_bucket doesn't increase when filling a cauldron with a water bucket.

Steps to Reproduce:

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

/scoreboard objectives add UseWaterBucket minecraft.used:minecraft.water_bucket
/scoreboard objectives setdisplay sidebar UseWaterBucket

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