I think we should be allowed to collect our remaining stuff after getting special rewards. 

Hello Gerhard.  A lot of us use the tactic to move to the next stage if we will find a hidden special offer for the specific stage. It is a common tactic but we might end  up having excessive keys, so you are not the only one having excessive keys. The feature is designed like this, and we know it, but still, we doing the very same mistake every time, myself included. The developers won't change the feature, but we can change our tactic 🙂