The method jsonSerialize() does not exist on DVDoug\BoxPacker\Item. It seems like you code against a sub-type of said class. However, the method does not exist in DVDoug\BoxPacker\ConstrainedPlacementItem or DVDoug\BoxPacker\Test\THPackTestItem. Are you sure you never get one of those?
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Ignorable by Annotation
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If this is a false-positive, you can also ignore this issue in your code via the ignore-call annotation