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