Some or all Challenge rewards are locked (their color is darker): you cannot tap them to see how many pieces of a reward you already have. Sometimes it's all of them in all tiers of a challenge, sometimes there are a few that are brighter and can still be tapped, but I cannot get this to work right now, so I only have a screenshot where all are locked and another where all are unlocked.
I've seen and reported a similar issue before. I could go around that by closing the challenges and reopening them, and
@CG_Kozispoon reported a fix in the update "RECENTLY FIXED BUGS 8/10". This time however, it seems more persistant: the only way to fix it (temporarily) is to restart the app.
As far as I'm aware, this reoccurred with the Ships update. Ship Challenges seem unaffected.
Locked:
And unlocked after restarting the game:
0