Join two lists in .NET while maintaining single property

I have two identical lists in .NET, this is how single item looks like:

public class InputRecord
{
    public string Product { get; set; }
    public int OriginYear { get; set; }
    public int DevelopmentYear { get; set; }
    public decimal IncValue { get; set; }
}

I'm trying to join two lists of these records using LINQ while updating IncValue where all other properties matches. My code looks like that:

var output = template.Join(records,
    t => new { t.Product, t.OriginYear, t.DevelopmentYear },
    r => new { r.Product, r.OriginYear, r.DevelopmentYear },
    (t, r) => { t.IncValue = r.IncValue; return t; }).ToList();

Template contains about 50 records, while records contains 12 entries. I expect the output to have 50 records with IncValue updated where all other properties matched, however my output is only 12 records. What I'm doing wrong?

Instead of using "join" which will only return matching items that have matching keys in both input sequences, can you try using the "group join" operation between two collections instead

var output = template
    .GroupJoin(
        records,
        t => new { t.Product, t.OriginYear, t.DevelopmentYear },
        r => new { r.Product, r.OriginYear, r.DevelopmentYear },
        (t, rs) => { 
            var record = rs.FirstOrDefault(); 
            if (record != null) { 
                t.IncValue = record.IncValue; 
            }
            return t; 
        })
    .ToList();

The GroupJoin method performs a left outer join by grouping the elements from the left list (template) with the matching elements from the right list (records).

I played around in .NET Fiddle using the following 2 collections

    // Define the template and records collections
    var template = new List<Item>()
    {
        new Item() { Product = "A", OriginYear = 2018, DevelopmentYear = 1, IncValue = 100 },
        new Item() { Product = "A", OriginYear = 2018, DevelopmentYear = 2, IncValue = 200 },
        new Item() { Product = "B", OriginYear = 2019, DevelopmentYear = 1, IncValue = 300 },
        new Item() { Product = "B", OriginYear = 2019, DevelopmentYear = 2, IncValue = 400 }
    };
    var records = new List<Item>()
    {
        new Item() { Product = "A", OriginYear = 2018, DevelopmentYear = 2, IncValue = 250 },
        new Item() { Product = "B", OriginYear = 2019, DevelopmentYear = 1, IncValue = 350 },
        new Item() { Product = "C", OriginYear = 2020, DevelopmentYear = 1, IncValue = 500 }
    };

The items in the output was

Product: A, OriginYear: 2018, DevelopmentYear: 1, IncValue: 100
Product: A, OriginYear: 2018, DevelopmentYear: 2, IncValue: 250
Product: B, OriginYear: 2019, DevelopmentYear: 1, IncValue: 350
Product: B, OriginYear: 2019, DevelopmentYear: 2, IncValue: 400

If this is not what you needed then I would probably suggest sharing some sample data.