In Bedrock Edition, the waterfall lands on the waterlogged block and continues flowing as if the waterlogged slab is a normal block. It's supposed to do what it does in Java Edition, which is to stop flowing because it landed on a waterlogged slab.
(In the Bedrock Edition screenshot, the hole is only there so the water doesn't flow everywhere)
This bug was fixed in Java Edition in 1.13-pre7: MC-127093
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Definitely could confirm this. I encountered this while building a kelp farm.
Ended up adding a row of trapdoors in the middle to block the waterflow:
[media]Without the trapdoor, the waterflow would look like something like this:
[media]However, in Java Edition, the water will not spilt out at all (I am using Java Edition 1.20.1 here, but it shouldn't make any difference) :
[media]