The docs state:

sparse and unique Properties

An index that is both sparse and unique prevents collection from having documents with duplicate values for a field but allows multiple documents that omit the key.

The idea of the [entire] key being absent has some nuance that is not immediately apparent; in the case of a compound key, all elements of the key must be absent in order for this to apply.

A suggested alternative would be:

allows multiple documents that omit all fields in the key