When using a RegEx in AutoFixture ISpecimenBuilder, Why do I always get back the same value?

My builder is set up to either deal with a Parameter or a Property. This may change in the future, but for now this is what I have in my builder:

public class UserNameBuilder : ISpecimenBuilder
{
    public object Create(object request, ISpecimenContext context)
    {
       var propertyInfo = request as PropertyInfo;
        if (propertyInfo != null && propertyInfo.Name == "UserName" && propertyInfo.PropertyType == typeof(string))
        {
            return GetUserName();
        }

        var parameterInfo = request as ParameterInfo;
        if (parameterInfo != null && parameterInfo.Name == "userName" && parameterInfo.ParameterType == typeof(string))
        {
            return GetUserName();
        }

        return new NoSpecimen(request);
    }

    static object GetUserName()
    {
        var fixture = new Fixture();
        return new SpecimenContext(fixture).Resolve(new RegularExpressionRequest(@"^[a-zA-Z0-9_.]{6,30}$"));
    }
}

My UserName object is a ValueType object and is as follows:

public class UserName : SemanticType<string>
{
    private static readonly Regex ValidPattern = new Regex(@"^[a-zA-Z0-9_.]{6,30}$");

    public UserName(string userName) : base(IsValid, userName)
    {
        Guard.NotNull(() => userName, userName);
        Guard.IsValid(() => userName, userName, IsValid, "Invalid username");
    }

    public static bool IsValid(string candidate)
    {
        return ValidPattern.IsMatch(candidate);
    }

    public static bool TryParse(string candidate, out UserName userName)
    {
        userName = null;

        try
        {
            userName = new UserName(candidate);
            return true;
        }
        catch (ArgumentException ex)
        {
            return false;
        }
    }
}

The UserName class inherits from SemanticType which is a project that provides a base for my value types.

Whenever I use AutoFixture as follows:

var fixture = new Fixture();
fixture.Customizations.Add(new UserNameBuilder());

var userName = fixture.Create<UserName>();

I always get the value "......" I thought I would get a different value each time. Is what I'm seeing expected behavior?

Thanks