AutoMapper inherintance with "new" modifier

Question

I'm updating my project from AutoMapper version 4.2.1 to 6 and, in the new version, I get an unexpected behaviour.

Classes

class Opportunity : OpportunityLite
{
}

class OpportunityLite
{
    public DealerOpportunityLite DealerOpportunity { get; set; }
}

class DealerOpportunity : DealerOpportunityLite
{
    public new string BranchCode { get; set; }
}

class DealerOpportunityLite
{
    public string BranchCode { get; set; }
}

Mapper Configuration

AutoMapper.Mapper.Initialize(cfg =>
{
    cfg.CreateMap<Opportunity, OpportunityLite>();
    cfg.CreateMap<DealerOpportunity, DealerOpportunityLite>();
});

Version: 4.2.1

If I execute the following Map:

Opportunity oppLite = new Opportunity
{
    DealerOpportunity = new DealerOpportunity
    {
        BranchCode = "00168"
    }
};

var opp = AutoMapper.Mapper.Map<Opportunity, OpportunityLite>(oppLite);

The property value of "opp.DealerOpportunity.BranchCode" is: 00168

From Version 5.0.2 to 6.2.2

In these versions the value of the same property is: null

How can I update AutoMapper to get the same result of version 4.2.1 without adding "ForMember" in the mapping configuration? Unfortunately, I have so many cases of this classes schema.