I'm not sure if this is how water is supposed to behave, but I observed this while toying around with 1.13-pre-2. For clarification, a block was placed underneath the flowing water blocks and water elevated, but did not return when block was removed.
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No it's not, non-full blocks just hide the water/lava faces, regardless of how the water was made; that's all MC-130486.
Blame the slab for hiding the water, you just commented on that report.