I can’t tell if this was intentional or it’s a bug. But it feels like a bug to me, as it’s caught me off guard a few times already.
Here’s a quick repro. If you drag in the new card element, go to the “Back Button” settings, and scroll down to Margin, at first the margins are un-linked with a value of 1em for the top. If you then type 0 in the value for top, it will automatically link the margins, so that if you immediately change the value to something else, they all change now.
Here’s a dropbox link to a video showing it in action. https://www.dropbox.com/s/pvkl5baqb92d6d6/Screen%20Recording%202019-07-26%20at%207.35.58%20PM.mov?dl=0