Just took a screenshot of it ingame: http://i.imgur.com/oZBgX.jpg
Since 5.1 I've started noticing this happening quite a few times to me. I sign up during Satchel bonus, it's gone when Invite pops up (Usually seconds after). I've even signed up during a Satchel bonus, had it show on invite - but didn't recieve on dungeon completion.
This is on EU. Is this happening on US aswell? Anyone heard of this being a bug or intentional?