It appeared to be caused by the direct sunlight.
The plastic used for marker body will deteriorate when illuminated with ultraviolet light over the long term.
To protect against ultraviolet damage, keep the product away from high temperatures and direct sunlight. Store the markers in a cool dark place.
Also, Copic ink is dye-based and strong light is absolute contraindication. This is another good reason the markers should not be exposed to direct rays of the sun.
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