Oh! I am sorry for this, I actually now tried one of my games and switched contect (choose and update async) and I now see the same as you do. Text is not mine, just a standard (localized) FB text.
Must have been in some late update of facebook! my code has been the same for months, now it works out like yours!
I know they changed the message for updateAsync Leaderboard, in october. Maybe they changed the updateAsync Custom as well...
Maybe you can ask in the Instant Games forum, or file a bug report
But as I said: in their offical github (which they uploaded one hour ago) they use this code. So maybe this bug is just temprary for today. (It happens a lot that some stuff break for a day or two and then came back)