Custom expression additional outputs don't work when connected to World Position Offset

Custom expression additional outputs are considered (0,0,0) when connected to WPO.

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1. create a custom node in a Material
2. add an additional output
3. set the additional output to a non-zero number and return this value as custom expression result too.
4. connect the custom expression's default output to WPO and it works.
5. connect the custom expression's additional output to WPO doesn't work.