Hi @Oliver Funk ,
Per my test, I could get the same result as yours on my end. Seems like it's by design that it would return the lookupId for lookupColumns in the response.
As a workaround, you could specify the return fields in the request like this: ?$expand=fields($select=lookupcolumn1,lookupcolumn2)
https://graph.microsoft.com/v1.0/sites/xyz.sharepoint.com,xxxxxxx/lists/yyyyyyyyyy/items?$expand=fields($select=id,Title,Jahr,Referat)
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