Constraint check results on references are hidden because references are collapsed by default. It is easy to overlook them in this case. Since we want to encourage people to clean up the violations we should make them more visible by expanding the references that have a constraint violation.
From the developer tools console, this can be done with the following command:
$( '.wikibase-referenceview .wbqc-reports-button' ) .parents( '.wikibase-statementview-references-container' ) .find( '.wikibase-statementview-references-heading .ui-toggler-toggle-collapsed' ) .click()
However, I hope our real implementation, if we decide to do this, will be a bit less hacky :)