The bug
You are unable to use a water bucket against a waterlogged block. This also relates to MC-126906 because it was a bug in java edition during development of the aquatic update, but the bug is still in bedrock Edition. For lava buckets it works fine.
How to reproduce:
Look with a water bucket on a waterlogged block.
Press right click.
What I observed: Nothing happened.
What I expected: Water gets placed next to, above or below the water logged block depending on which side of the block you are looking. Like it happens with lava buckets.
Affects 1.16.0