Can confirm for 1.14 Pre-Release 5.
The glitch appears to have started in 19w03a.
This, if left in the game, would break a lot of commands.
Is present in 18w22b
Huh. That's strange. I was sure I had double checked for duplicates.
Oh well. A bug is a bug.
This issue is not a duplicate. It still remains unsolved.
I am not stating that flowing water acts like air. It behaves entirely like normal water, just except that you can't swim in it unless you enter from a source block.
Wow, just realized this was a duplicate.
Is it just that "at" does not set @s to the previous use of @a in the command? If so, is this a bug?
EDIT - ah, yes. Here it is in the changelog.
/execute at <entity> <chained command> executes a command using the position of <entity> (but doesn't change entity)
https://gist.github.com/Dinnerbone/943fbcd763c19be188ed6b72a12d7e65
/execute at @a run give @s minecraft:cookie OR /execute at @a run tp @s ~ ~1 ~
in a command block
an interesting bug, to say the least. I hope it gets patched soon.