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A Forward Start Option may be created with any type of striked payoff, since the corresponding QuantLib type, the ForwardVanillaOption, places no such restriction.
Nevertheless QuantLib calculates the price assuming the payoff type is Vanilla, even if it is otherwise.
Click on download to download an xml file that contains an Excel formula that demonstrates this issue.
You may then reproduce the issue in spreadsheet by clicking on Go -> Load Excel Formula from Text File or one of its variations and choose the xml file downloaded in the step above.