https://lists.oasis-open.org/archives/ubl-comment/201712/msg00001.html

The element cac:TenderResult/cac:WinningParty/cbc:Rank assumes that the Rank information is unique for all lots to which the tenderer has submitted a bid. The Lot information can be found only in the cac:TenderResult/cac :Contract also assumes that a Contract element pre exists the Contract Award Notice to be able to provide lot information.

The same approach as the Call ForTenders should be taken.
Have a cac ProcurementprojectResult in the root that is the composition of an optional element cac:ProcurementprojectResult /cac:ProcurementprojectLot (0…n) and cac:TenderResult would allow to manage different results per Lot for one tenderer.