The bug

You can’t use water or lava buckets while aiming at a block that gets destroyed by water or lava. What should happen is that you replace the block with water or place it adjacent to it. I have tested it with tall grass, double tall grass, crops, torches etc. This is also related to MCPE-59107, where you can’t use buckets against waterlogged blocks.

How to reproduce:

1. Place a flower, for example sunflower
2. Try to use a water or lava bucket while aiming at the sunflower

Expected result: Water will be placed adjacent to the block.
Actual result: Nothing happened upon right clicking.