I am able to replicate same scenario. As per this Query Parameters documentation "$expand is not currently supported with advanced queries."
(endsWith) operators on the $filter is considered as Advanced query that is why expand is not working in the above Graph API.
As a workaround you can use
https://graph.microsoft.com/v1.0/users?$count=true&$top=13&$select=id,givenName&$filter=startsWith(mail,'Alex')&$expand=manager($select=id,givenName) this Graph API endpoint. Please refer to the below sample screenshot.
Hope this helps.
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