I actually know why. I ran into this problem when creating a command thing. I think they are editing the rotation at which the calculation for arrow motion is being fired at, rather than the actual arrow motion. Let's say they are editing the arrow so it is going to the left, if you look straight down and move your mouse left, you're still looking straight down. If they actually applied a leftward motion on it (which is what I did for my commands), it would probably work properly.
It does happen in 1.12
not sure, haven't checked
Oh, thanks for the fix.
I tried It without OptiFine, Didn't work again
confirmed for 1.13
This is now an issue in Java edition 1.13, not sure if the two issues are related.