Outlook addin commands fail on Windows Desktop client - outlook

Question

I have created an addin for outlook that works as expected on Web, Windows and MacOS. But recently i have introduced Controls such as TaskPane Menu Buttons, Which on click invoke a function.
In Manifest.xml
<Control xsi:type="Button" id="msgComposeFunctionButton">
<Label resid="funcComposeButtonLabel" />
<Supertip>
<Title resid="funcComposeSuperTipTitle" />
<Description resid="funcComposeSuperTipDescription" />
</Supertip>
<Icon>
<bt:Image size="16" resid="InfoIcon.16x16" />
<bt:Image size="32" resid="InfoIcon.32x32" />
<bt:Image size="80" resid="InfoIcon.80x80" />
</Icon>
<Action xsi:type="ExecuteFunction">
<FunctionName>addBccToComposeEmail</FunctionName>
</Action>
</Control>
In Commands.js
function addBccToComposeEmail(event) {
Office.context.mailbox.item.bcc.setAsync(['test.test#test.com']);
event.completed();
}
This works as expected on MacOS and Web, but shows a loader message in Windows desktop client which never stops.
Spinning loader image
Kindly let me know what the issue might be ?
#Outlook Team pls help here.
#Office JS team, Can u kindly respond to this question ?