I've included a download world, as it's a little difficult to explain. But it seems in Adventure Mode, you can't pickup water with a bucket unless you are 3 or more blocks away. It could be angle related.

This problem doesn't exist in Creative or Survival.

My world shows the lime blocks that allow you do pull water from a coordinate. Anything closer, does not allow you to.

Step 1: Create a "Redstone Ready" superflat world
Step 2: Issue these two commands on repeating command blocks.

Commands I'm using:

replaceitem entity @a[nbt={SelectedItem:{id:"minecraft:bucket"}},nbt=!{SelectedItem:{id:"minecraft:bucket",tag:{CanPlaceOn:["minecraft:water"]}}}] weapon.mainhand bucket{CanDestroy:["minecraft:water"]}

replaceitem entity @a[nbt={SelectedItem:{id:"minecraft:water_bucket"}},nbt=!{SelectedItem:{id:"minecraft:water_bucket",tag:{CanPlaceOn:["minecraft:sandstone"]}}}] weapon.mainhand water_bucket{CanPlaceOn:["minecraft:sandstone"]}

I don't recall this issue being present in 1.12