The only speculation I have on that front brings up more questions than it does answers.

Namely: that we couldn't actually detect the corruptive magical signature of the moonstone in the chimera until it was near dead and burning itself out. But if the chimera as a vessel masked the signature of the moonstone while alive, and the moonstone in turn masked the signature of the other wizards, the question then becomes how, exactly, did the other wizards somehow find themselves tied to the stones?