Ok, but even if I add @Valid in front of the dto, I am getting:

org.springframework.beans.InvalidPropertyException: Invalid property 'consumerGroups[0]' of bean class [...ApplicationDTO]: Illegal attempt to get property 'consumerGroups' threw exception; nested exception is org.springframework.beans.InvalidPropertyException: Invalid property 'consumerGroups[0]' of bean class [...ApplicationDTO]: Property referenced in indexed property path 'consumerGroups[0]' is neither an array nor a List nor a Set nor a Map; returned value was [Optional[[ConsumerGroupDTO(consumerGroupId=, simpleConsumerGroup=false)]]]

This seems like a bug, it doesn’t generate a proper constraint violation.