Program.cs

using ZiggyCreatures.Caching.Fusion;
using ZiggyCreatures.Caching.Fusion.Serialization.SystemTextJson;

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddFusionCache()
    .WithDefaultEntryOptions(options => { options.EnableAutoClone = true; })
    .WithSerializer(new FusionCacheSystemTextJsonSerializer());

var app = builder.Build();

app.MapGet("/single", async (IFusionCache cache) =>
{
    var result = new Participant();
    var user = new User
    {
        ID = 200,
        Name = "Another User"
    };

    // Retrieve participant from cache
    var cachedParticipant = await cache.TryGetAsync<Participant>(
        $"participant:4",
        options => { options.EnableAutoClone = true; });

    if (!cachedParticipant.HasValue)
    {
        // Save participant to cache if missing
        var participant = new Participant
        {
            ID = 1,
            UserID = user.ID
        };
        await cache.SetAsync($"participant:4", participant, options => { options.EnableAutoClone = true; });
    }
    else result = cachedParticipant.Value;

    // Set user object on cached participant
    result.User = user;
    /*
     * The user object here is null, which shows Auto Clone is working as expected for objects not in a list.
     */

    return Results.Ok(result);
});

app.MapGet("/list", async (IFusionCache cache) =>
{
    var results = new List<Participant>();
    var user = new User
    {
        ID = 100,
        Name = "A User"
    };

    // Retrieve participants from cache
    for (int i = 1; i <= 3; i++)
    {
        var cachedParticipant = await cache.TryGetAsync<Participant>(
            $"participant:{i}",
            options => { options.EnableAutoClone = true; });
        if (cachedParticipant.HasValue)
            results.Add(cachedParticipant.Value);
    }

    if (results.Count < 3)
    {
        // Generate 3 participants
        var participants = new List<Participant>();
        for (int i = 1; i <= 3; i++)
        {
            participants.Add(new Participant
            {
                ID = i,
                UserID = user.ID
            });
        }

        // Store participants in cache
        foreach (var participant in participants)
        {
            await cache.SetAsync($"participant:{participant.ID}", participant,
                options => { options.EnableAutoClone = true; });
        }

        results.AddRange(participants);
    }

    // Set user object on cached participants
    foreach (var participant in results)
        /*
         * `results` entries on the 2nd request here though have the `User` set on them despite Auto Clone
         * being enabled.
         */
    {
        participant.User = user;
    }

    return Results.Ok(results);
});

app.Run();

class Participant
{
    public int ID { get; set; }
    public int UserID { get; set; }
    public User User { get; set; }
}

class User
{
    public int ID { get; set; }
    public string Name { get; set; }
}