Items don't float up in falling water

Any way to reproduce this consistently?

Dig up some sand under water, and try not to collect them. Some will stay at the bottom

Wouldn't that be because items can't flow up the falling water that replaces it?

Yup, there's no water source blocks at those locations, so the items can't float up (as the water is flowing downward).

But the water probably would've gotten filled in to make a water source. In the image, it looks like there's probably a source there. 

"Falling water" 

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