OnGetAsync is called, no error is thrown, but output is wrong, and breakpoints are being skipped

I'm trying to make some modifications to the DotNetCoreRazor_MSGraph sample project so that instead of just presenting mails, regardless of folder, it first creates the folder structure.

This is the code for EmailFolders.cshtml:

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using DotNetCoreRazor_MSGraph.Graph;
using Microsoft.AspNetCore.Authorization;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.RazorPages;
using Microsoft.Extensions.Logging;
using Microsoft.Graph;
using Microsoft.Identity.Web;
using Newtonsoft.Json;

namespace DotNetCoreRazor_MSGraph.Pages
{
    [AuthorizeForScopes(ScopeKeySection = "DownstreamApi:Scopes")]
    public class EmailFoldersModel : PageModel
    {
        private readonly GraphEmailClient _graphEmailClient;
        private readonly GraphServiceClient _graphServiceClient;
        private string inboxid = null;
        private string sentitemsid = null;
        private string draftsid = null;

        [BindProperty(SupportsGet = true)]
        public string NextLink { get; set; }
        public IEnumerable<Message> Messages  { get; private set; }

        public EmailFoldersModel(GraphEmailClient graphEmailClient, GraphServiceClient graphServiceClient)
        {
            _graphEmailClient = graphEmailClient;
            _graphServiceClient = graphServiceClient;
        }

        public StringBuilder sb = new StringBuilder();
        public List<MailFolder> folders = new List<MailFolder>();

        public async Task OnGetAsync()
        {
            MailFolder inbox = await _graphServiceClient.Me.MailFolders.Inbox.Request().GetAsync();
            inboxid = inbox.Id;
            MailFolder sentitems = await _graphServiceClient.Me.MailFolders.SentItems.Request().GetAsync();
            sentitemsid = sentitems.Id;
            MailFolder drafts = await _graphServiceClient.Me.MailFolders.Drafts.Request().GetAsync();
            draftsid = drafts.Id;
            
            //get all folders on all pages
            IUserMailFoldersCollectionPage flds = await _graphServiceClient.Me.MailFolders.Request().GetAsync();
            folders.AddRange(flds.CurrentPage);
            while (flds.NextPageRequest != null)
            {
                flds = await flds.NextPageRequest.GetAsync();
                folders.AddRange(flds.CurrentPage);
            }

            //move drafts to the top of the list
            foreach (MailFolder folder in folders)
            {
                if (folder.Id == draftsid)
                {
                    folders.Remove(folder);
                    folders.Prepend(folder);
                    break;
                }
            }
            //move sentitems to the top of the list
            foreach (MailFolder folder in folders)
            {
                if (folder.Id == sentitemsid)
                {
                    folders.Remove(folder);
                    folders.Prepend(folder);
                    break;
                }
            }
            //move inbox to the top of the list
            foreach (MailFolder folder in folders)
            {
                if (folder.Id == inboxid)
                {
                    folders.Remove(folder);
                    folders.Prepend(folder);
                    break;
                }
            }

            //create the HTML <ul> structure
            sb.AppendLine("<ul id=\"myUL\">");
            foreach (MailFolder folder in folders.Where(f => f.ParentFolderId == null))
            {
                if (folder.ChildFolderCount > 0)
                    sb.AppendLine("<li class=\"caret\">");
                else
                    sb.AppendLine("<li>");
                sb.AppendLine(folder.DisplayName);
                if (folder.ChildFolderCount > 0)
                {
                    await CreateList(folder.Id);
                }
                sb.AppendLine("</li>");
            }
            sb.AppendLine("</ul>");
            await Task.CompletedTask;
        }

        //html recursor
        private async Task CreateList(string MailFolderId)
        {
            StringBuilder sb = new StringBuilder();
            sb.AppendLine("<ul class=\"nested\">");
            foreach (MailFolder folder in folders.Where(f => f.ParentFolderId == MailFolderId))
            {
                if (folder.ChildFolderCount > 0) 
                    sb.AppendLine("<li class=\"caret\">");
                else
                    sb.AppendLine("<li>");
                sb.AppendLine(folder.DisplayName);
                if (folder.ChildFolderCount > 0)
                {
                    await CreateList(folder.Id);
                }
                sb.AppendLine("</li>");
            }
            sb.AppendLine("</ul>");
        }

        //folder recursor
        private async Task GetChildFolders(string MailFolderId)
        {
            List<MailFolder> childfolders = new List<MailFolder>();

            IMailFolderChildFoldersCollectionPage chlds = await _graphServiceClient.Me.MailFolders[MailFolderId].ChildFolders.Request().GetAsync();
            childfolders.AddRange(chlds.CurrentPage);
            while (chlds.NextPageRequest != null)
            {
                chlds = await chlds.NextPageRequest.GetAsync();
                childfolders.AddRange(chlds.CurrentPage);
            }

            folders.AddRange(childfolders);

            foreach (MailFolder folder in childfolders)
            {
                if (folder.ChildFolders.Count > 0)
                {
                    await GetChildFolders(folder.Id);
                }
            }
        }
    }
}

On the EmailFolders.cshtml page, I'm trying to grab the folder structure with @Html.Raw(Model.sb.ToString()). I know OnGetAsync is being called, because the result is <ul id="myUL"></ul> (with carriage returns after each tag). But I'm trying to see why nothing is being added to the folders list, and it's bypassing any breakpoints I use. Breakpoints on EmailFolders.cshtml work, but that doesn't help, because it's after the codebehind is finished.