Why is Automapper or automapper.data behaving differently in .NET framework and .NET 6?

Question

I have a Web API that was written in .NET 4.6.1 and I want to move it to .NET 6.

I use Automapper.Data to map SQL data reader to a C# object and found a strange behaviour.

Say this is the output object:

public class PriceInfoV11
{
    public List<Price> Prices { get; set; } = new List<Price>();
}    

public class ThunderPriceInfoV11 : PriceInfoV11
{
    [JsonIgnore]
    public double? BidRaw { get; set; }
}

We can see that ThunderPriceInfoV11 inherits from PriceInfoV11 and has two properties, BidRaw and a list of Price objects.

When AutoMapper was running in .NET 4.6 (or 4.8), the following config works when mapping an IDataReader to ThunderPriceInfoV11:

var config = new MapperConfiguration(cfg =>
            {
                cfg.AddDataReaderMapping();
                cfg.CreateMap<IDataReader, ThunderPriceInfoV11>()
                    .ForMember(fu => fu.BidRaw,
                        u => u.MapFrom(um =>
                            um.IsDBNull(um.GetOrdinal("Bid")) ? (double?)null : (double)um.GetDecimal(um.GetOrdinal("Bid"))))
                    ;
            });
_mapper = config.CreateMapper(); 

However, in .NET 6, this same config throws an error saying it doesn't know property Prices.

To make it work, I need to add:

.ForMember(fu => fu.Prices, opt => opt.Ignore())

I am bit confused on why the same version of AutoMapper and AutoMapper.Data behaves differently in .NET 4.8 and .NET 6. I tried the latest version of AutoMapper and an older version, and this behaviour is the same in both.