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It was already answered in the comments that the issue is a Lab cache that you found on one day (without any normal cache). The Challenge cache listing does not explicitly allow or denies lab cache finds so by default these should be included in challenges like this. The checkers already have a way to retrive lab cache data:
So I would recommend you to open a new checker request in https://project-gc.com/forum/list?8 and after it will be in, ask the original tagger of the previously posted checker (Aix sponsa) or anyone with moderator privileges to disable/remove the old one.
There will be thousands of checkers like this to fix as this API is relatively new and most of the old checkers was not updated.