How to replicate the bug:

1. Set your game mode to adventure mode:
/gamemode adventure @p

2. Get a water bucket with the CanPlaceOn tag, for example one that can only be placed on stone:
/give @p water_bucket{CanPlaceOn:[stone]}
You can check it's correctness by hovering over the item and seeing that it actually correctly registerd the tag (no typo's). See first attached screenshot.

3. Hold the water bucket and look at a stone block. The block is outlined, because the bucket has the CanPlaceOn tag for it. See second attached screenshot.

4. Hold the water bucket and look at another block. The block is not outlined, because the bucket does not have the CanPlaceOn tag for it. See third attached screenshot.

5. Place the water on that other (not-stone) block. Even though the block wasn't outlined, the water will still be placed. See fourth attached screenshot.

EDIT:

• If there is no CanPlaceOn tag at all on the water bucket, you can still place it on any block.
• The same goes for lava buckets.
• Water and lava can also be picked up with any bucket (no need to specify the liquid in a CanPlaceOn tag).