Normally, margin top and margin bottom resolve against the width of the containing block, but the working group decided that for flex items, we should resolve against the height of the containing block.
This should be an easy change to make. See RenderFlexibleBox::computeChildMarginValue.
*** Bug 120003 has been marked as a duplicate of this bug. ***
*** Bug 126809 has been marked as a duplicate of this bug. ***
Both Edge & Firefox teams have committed to change to align with the Chrome/Safari behavior. The spec also got an update to mandate this behavior: https://drafts.csswg.org/css-grid-1/#item-margins
It sounds that this issue should be closed as "Won't Fix".
Right -- and the spec changed, too, such that WebKit is now correct according to the spec.
So really this could now be "Resolved|Invalid", in the sense that WebKit is matching what the spec requires, i.e. the reported behavior is actually correct/intentional & not a bug.
(At least, that'd be the correct resolution if this were in the Mozilla/Firefox bug tracker -- I'm not sure if the bug-resolution semantics are the same in WebKit's bug tracker.)