I don't think there is anything unusual about the recipient of an honour not knowing about it until close to the announcement.
Of course someone will be identified and selected for their nomination, and that can be without their knowledge.
The accusation seems to be that she was identified for a peerage before making her report. As long as she had no knowledge of that, then there is no impropriety. There is no proof, only suspicion, that she had prior knowledge of her peerage.
Saying that, if you are going to conduct an 'independent' report, it is barmy to select someone that you plan to elevate to the Lords sooner, rather than later. But that is Corbyn for you.