Hello @Suresh Babu Patike
If they were originally the on-prem users, the onPremisesExtensionAttributes attribute authority is Exchange in which case MS Graph wont be able to fetch them.
This seems to be expected behavior. Please check if this scenario is the same for you. https://learn.microsoft.com/en-us/answers/questions/456452/cant-edit-extension-attributes-with-graph-api
Regards
Shaofan
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