I think it'll also be failing in case when validator is declared in javax.validation.ConstraintValidator. Maybe we can change

return CollectionHelper.asSet(
    ConstraintCheckIssue.error(
        element,
        constraintMirror,
        "CONSTRAINT_TYPE_WITHOUT_VALIDATOR"
    )
);

to use ConstraintCheckIssue.warning instead of error? And change a message to something like "no validator was declared in validatedBy parameter, make sure that it is declared in either javax.validation.ConstraintValidator, or programmatically, or in XML"...