DefaultMessage not set for unique field validation in unit tests

package defaultmessage

class User {
    
    String username

    static constraints = {
        username(unique:true)
    }
}

package defaultmessage

import grails.test.mixin.*
import org.junit.*

@TestFor(User)
@Mock(User)
class UserTests {
    
    void setUp() {
        new User(username:"test").save()
        println User.list()
    }

    void tearDown() {
        User.deleteAll()
    }

    void testUniqueErrorMessage() {
       def user = new User(username:"test")
       assert user.validate() == false
       assert user.errors['username'].defaultMessage == "Property [{0}] of class [{1}] with value [{2}] must be unique"
    }
}

| Failure:  testUniqueErrorMessage(defaultmessage.UserTests)
|  Assertion failed: 

assert user.errors['username'].defaultMessage == "Property [{0}] of class [{1}] with value [{2}] must be unique"
       |    |     |            |              |
       |    |     |            null           false
       |    |     Field error in object 'defaultmessage.User' on field 'username': rejected value [test]; codes [defaultmessage.User.username.unique.error.defaultmessage.User.username,defaultmessage.User.username.unique.error.username,defaultmessage.User.username.unique.error.java.lang.String,defaultmessage.User.username.unique.error,user.username.unique.error.defaultmessage.User.username,user.username.unique.error.username,user.username.unique.error.java.lang.String,user.username.unique.error,defaultmessage.User.username.unique.defaultmessage.User.username,defaultmessage.User.username.unique.username,defaultmessage.User.username.unique.java.lang.String,defaultmessage.User.username.unique,user.username.unique.defaultmessage.User.username,user.username.unique.username,user.username.unique.java.lang.String,user.username.unique,unique.defaultmessage.User.username,unique.username,unique.java.lang.String,unique]; arguments [username,class defaultmessage.User,test]; default message [null]
       |    org.grails.datastore.mapping.validation.ValidationErrors: 1 errors
       |    Field error in object 'defaultmessage.User' on field 'username': rejected value [test]; codes [defaultmessage.User.username.unique.error.defaultmessage.User.username,defaultmessage.User.username.unique.error.username,defaultmessage.User.username.unique.error.java.lang.String,defaultmessage.User.username.unique.error,user.username.unique.error.defaultmessage.User.username,user.username.unique.error.username,user.username.unique.error.java.lang.String,user.username.unique.error,defaultmessage.User.username.unique.defaultmessage.User.username,defaultmessage.User.username.unique.username,defaultmessage.User.username.unique.java.lang.String,defaultmessage.User.username.unique,user.username.unique.defaultmessage.User.username,user.username.unique.username,user.username.unique.java.lang.String,user.username.unique,unique.defaultmessage.User.username,unique.username,unique.java.lang.String,unique]; arguments [username,class defaultmessage.User,test]; default message [null]
       defaultmessage.User : null

	at defaultmessage.UserTests.testUniqueErrorMessage(UserTests.groovy:26)
| Completed 1 unit test, 1 failed in 2225ms
| Packaging Grails application.....
| Tests FAILED  - view reports in target/test-reports
| Error Error running script test-app -echoOut: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'transactionManagerPostProcessor': Initialization of bean failed; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'transactionManager': Cannot resolve reference to bean 'sessionFactory' while setting bean property 'sessionFactory'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'sessionFactory': Invocation of init method failed; nested exception is java.lang.ClassCastException: org.grails.datastore.gorm.validation.constraints.UniqueConstraint cannot be cast to org.codehaus.groovy.grails.orm.hibernate.validation.UniqueConstraint (Use --stacktrace to see the full trace)

import defaultmessage.User
new User(username:"test").save()

def user = new User(username:"test")
assert user.validate() == false
assert user.errors['username'].defaultMessage == "Property [{0}] of class [{1}] with value [{2}] must be unique"