Thanks, this looks like a real bug. FWIW it's not specific to primary constructors or records, this fails too:

using System.Text.Json.Serialization;
using System.Text.Json;

var singleUserJson = """{"Username":"Filip","PhoneNumbers":["123456"]}""";
var userFromJson = JsonSerializer.Deserialize<User>(singleUserJson);
Console.WriteLine(userFromJson.PhoneNumbers.Count); // 0

public class User
{
    public User(string name)
        => Name = name;

    public string Name { get; }

    [JsonObjectCreationHandling(JsonObjectCreationHandling.Populate)]
    public List<string> PhoneNumbers { get; } = new();
}

Originally posted by @eiriktsarpalis in dotnet/docs#37329 (comment)