The Bug:

minecraft.used:minecraft.potion doesn't increase when filling a partially filled cauldron with a water bottle.

Steps to Reproduce:

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

/scoreboard objectives add UseWaterBottle minecraft.used:minecraft.potion
/scoreboard objectives setdisplay sidebar UseWaterBottle

• Fill a cauldron with some water using a water bottle.
  →  Notice how the scoreboard increases.
• Fill the cauldron again with the water bottle.
  →  Notice how the scoreboard doesn't increase.
• Fill the cauldron again for the final time with the water bottle.
  →  Notice how the scoreboard doesn't increase.