https://github.com/MicrosoftDocs/windows-powershell-docs/blob/master/docset/windows/dhcpserver/get-dhcpserverv4reservation.md
when use
Get-DhcpServerv4Reservation -ScopeId $ScopeId -ComputerName $DHCPServer -ClientId $ClientId
it returns the leased (not reserved) client IP if the ClientID exists and is active in the given scope
There is a workaround = collect all reservations for the given scope and search in the result against ClientID