The main function mintReceipt responsible for minting receipts lacks an important check to ensure the quest end time hasnt finished yet. Considering the fact that on quest creation every quest is enforced with a startTime and endTime, which represents the quest starting time and ending time. Users should not be allowed to mint receipts after the quest is expired.
By the sponsor comment, the claimSignerAddress takes care of that on the off-chain side and wont issue hashes before the quest start or after the quest ends. But mistakes always can occur and it is recommended to have a check on the smart contract level as well.
Here is a recommended change, which takes care of this problem:
