Hello Shaikh Shahrukh,
Thank you for contacting Microsoft!
For the query of retrieving all assigned/planned tasks using Microsoft Graph API with a single call, Unfortunately there is no way for the mentioned requirement. I completely understand the impact it is causing and apologies for the inconvenience. I would recommend You can submit this feature request using this support link, which will be monitored by Microsoft team and make the enhancements to Graph API
Hope this helps.
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