the definition of a bug:
"A software bug is an error, flaw or fault in a computer ** program or system that causes it to produce an incorrect or unexpected result, or to behave in unintended ways."
This behaviour is expected as it is consistent with all previous versions going back to beta version 1.8.
https://mcupdate.tumblr.com/post/10199253666/minecraft-beta-18
Just because it surprised you does NOT mean that it is not WAI.
Not a bug
I think this is WAI, all other pushable, non-full blocks (trapdoors fence gates, walls, fences end rods ect..) act the same way.
Feature request, not a bug.
I also observed this with one important additional observation. The amount of time taken for the sand/Anvil/gravel to dissapear while falling is identical to the time taken for lit TNT to explode. I suspect that the two are linked.
(Also on iOS )
I've attached some images of a suitable sand cannon that can be used to show this bug (the sand never returns)
Pretty sure this is WAI