It looks like that’s exactly what they did. Up until the update for the Bastet challenge I had all of the special achievements unlocked, so I was very confused when I played a game of CTF in the arcade and got the popup for an achievement when I captured a flag.
As you indicated, there isn’t any text specifying what the reward is, and the spot where you would normally see the reward spray is just a solid dark-blue box.
CTF is currently in the weekly arcade slot, so they’ll probably end up fixing this (as opposed to simply withholding the CTF game-type until the new year event rolls around).
I’ll add an update when the event starts; either I’ll unlock the spray immediately when the event actually goes live, or the achievement will be redacted and I’ll have to earn it again.
UPDATE
Now that the event is live, the achievements still show as unlocked, and I can see the sprays on the achievement tiles, but the sprays are locked in my hero gallery.
Blizzard has confirmed they are aware of the bug and are working on a fix per the following thread: