RavenDB 4.2 Map-Reduce dictionary

Question

I've got a collection of chat messages, which have a list of guids which is the members who have seen this message.

To be able to check the unread message count per chat, i aggregate these results by checking how many messages there are vs how many messages the user have seen.

Originally i came up with this index:

public class ChatMessages_BySeen : AbstractIndexCreationTask<ChatMessage, ChatMessages_BySeen.Result>
{
    public override string IndexName => nameof(ChatMessages_BySeen);

    public class Result
    {
        public string GroupId { get; set; } = null!;
        public IEnumerable<Guid> SeenBy { get; set; } = null!;
        public int Total { get; set; }
    }

    public ChatMessages_BySeen()
    {
        Map = messages => 
                            from message in messages
                            select new
                            {
                                GroupId = message.GroupId,
                                SeenBy = message.SeenBy.Select(x => x),
                                Total = 1
                            };

        Reduce = results =>
                            from result in results
                            group result by result.GroupId into g
                            select new
                            {
                                GroupId = g.Key,
                                SeenBy = g.SelectMany(x => x.SeenBy.Select(p => p)),
                                Total = g.Sum(x => x.Total)
                            };
    }
}

But this quickly generates a large list of duplicate user ids. So instead i thought, with some inspiration from https://stackoverflow.com/a/17499412/11388196 that i could just aggregate the count of each member ->

    public class ChatMessages_BySeen : AbstractIndexCreationTask<ChatMessage, ChatMessages_BySeen.Result>
{
    public override string IndexName => nameof(ChatMessages_BySeen);

    public class Result
    {
        public string GroupId { get; set; } = null!;
        public Dictionary<Guid, int> SeenCounter { get; set; } = null!;
        public int Total { get; set; }
    }

    public ChatMessages_BySeen()
    {
        Map = messages => 
                            from message in messages
                            select new
                            {
                                GroupId = message.GroupId,
                                SeenBy = message.SeenBy.ToDictionary(x => x, x => 1),
                                Total = 1
                            };

        Reduce = results =>
                            from result in results
                            group result by result.GroupId into g
                            select new
                            {
                                GroupId = g.Key,
                                SeenBy = g.SelectMany(x => x.SeenCounter).GroupBy(x => x.Key).OrderBy(x => x.Key).ToDictionary(x => x.Key, x => x.Sum(y => y.Value)),
                                Total = g.Sum(x => x.Total)
                            };
    }
}

this however, always sets the SeenCounter to be empty/null. Can anyone help me in the right direction?