Hi @Ayase Morita,
Per my test, The createdBy and lastModifiedBy properties will return user and owner usually returns group. Since the list owner is same with the site owner, and the site owner always be created as owner group as default. So the owner usually return group. Here is my library permission and graph response
For owner and lastModifiedBy properties not returned, as explained in the documentation, the Graph API will return null values for attributes it does not have permissions to read:
When an application queries a relationship that returns a directoryObject type collection, if it doesn't have permission to read a certain resource type, members of that type are returned but with limited information. For example, only the @odata.type property for the object type and the id is returned, while other properties are indicated as
null
. With this behavior, applications can request the least privileged permissions they need, rather than rely on the set of Directory.* permissions. For details, see Limited information returned for inaccessible member objects.
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