Maybe not on all cruises, but on most. It is absolutely logic that MSC needs to limit the bookings to allocation. Otherwise loads of cabins are not bookable because they are occupied one or two days. Late releases for YC cabins were very common, now with the upgrade bidding less.
The Virtuosa cruise you are talking counts as five different cruises that share one boat. You cannot grab a Rotterdam cabin from Southampton. Only if it is empty for the next cruise also, then there will be a late release.