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The PayAtExpiry setting of american options seems to be ignored. Apparently the payoff realised at exercise time is aqssumed to be paid immediately regardless of the PayAtExpiry setting.
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 XML or one of its variations and choose the xml file downloaded in the step above.