Water bottles (minecraft:potion) have a different NBT tag depending on how you obtain them, the NBT tags are as follows:

• from creative:

  {Potion:"minecraft:water"}

• from cauldron (survival): (no NBT tag)

  {}

• from water (survival/creative):

  {Potion:"minecraft:water"}

This bug makes trading with villagers inconvenient, as when you specify the tag:

{Potion:"minecraft:water"}

You will likely find that players use cauldrons to obtain the water bottle, and the trade fails.

To fix this bug, all that needs to happen is to make any potion without a tag, be assigned the default water bottle tag, or at least when obtained using non-command methods. Cauldrons and water sourceblocks should yield the player the exact same water bottle.

[Attachments (in order): "cauldron", "creative", "water-source"]