@Jemima yes this has been discussed on the forum before , here is a recent thread ( from two weeks ago )about the same issue . Although in this case the membership expired a day after the sit but they still couldn’t write a review .
https://forum.trustedhousesitters.com/t/membership-cancelled-day-after-sit-cannot-review-sitters
Was this sit confirmed before Sep 23 ?
According to previous post by admin there is supposedly a feature in place since Sep 23 that should prevent a member from confirming a sit that goes beyond your membership expiry date - unless you have opted into auto renew . Also it is supposed to prevent you from turning off auto renew until you have officially cancelled the sits on the dashboard.
This doesn’t seem to have worked in your case .