Specification texts are taken from the W3C specification W3C CSS Flexible Box Layout Module
Own comments and notes in blue color.

Flexbox: margin

Auto margins on flex items have an effect very similar to auto margins in block flow:

During calculations of flex bases and flexible lengths, auto margins are treated as ‘0’.
Prior to alignment via ‘justify-content’ and ‘align-self’, any positive free space is distributed to auto margins in that dimension.
Overflowing elements ignore their auto margins and overflow in the end/foot direction.

Note that, if free space is distributed to auto margins, the alignment properties will have no effect in that dimension because the margins will have stolen all the free space left over after flexing.

Demos

margin: auto;

margin-left: auto at second item

margin-left: auto at second and fourth item

Lorem ipsum dolor sit amet,

margin-top: auto at last item

margin-top: auto at last item with align-items:flex-start

// here: create two groups | flex-direction is column
// same result as above - the align-items property has no effect because margin-top: auto has stolen the free space