Accessing Data on the interface Graze\CiffRenderer\Simpl...mpleXmlElementInterface suggest that you code against a concrete implementation. How about adding an instanceof check?
The method xpath() does not exist on Graze\CiffRenderer\Simpl...mpleXmlElementInterface. It seems like you code against a sub-type of Graze\CiffRenderer\Simpl...mpleXmlElementInterface such as Graze\CiffRenderer\Simpl...lement\SimpleXmlElement.
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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