We assume that hard constraints are supposed to be enforced even when edits are reverted (or when an old revision is restored, which is just an undo under the hood), whereas soft constraints aren’t. It would be good to document this in the Wikibase: Constraints documentation. (And maybe mention that these constraints are unrelated to Wikibase-Quality-Constraints, while we’re at it.)
NOTE: It probably makes sense to wait with this until T289473: Undo/revert allows having several properties with the same label in the same language is done.