Disable the `AuthenticationHandler`'s new behavior of setting the default authentication scheme to the single scheme registered when there is only one

Program.cs

using Microsoft.AspNetCore.Authentication;
using Microsoft.Extensions.Options;

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.
// Learn more about configuring Swagger/OpenAPI at https://aka.ms/aspnetcore/swashbuckle
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

builder.Services.AddAuthentication().AddJwtBearer();

if (builder.Services.FirstOrDefault(r => r.ServiceType == typeof(IAuthenticationSchemeProvider)
                                         && r.ImplementationType == typeof(AuthenticationSchemeProvider)) is not null)
{
    // Default AuthenticationSchemeProvider is registered, override that with a wrapper that disables its behavior of setting the
    // default authentication scheme when only one scheme is registered.
    builder.Services.AddSingleton<IAuthenticationSchemeProvider, DisableAutoDefaultSchemeAuthenticationSchemeWrapper>();
}

var app = builder.Build();

// Configure the HTTP request pipeline.
if (app.Environment.IsDevelopment())
{
    app.UseSwagger();
    app.UseSwaggerUI();
}

app.UseHttpsRedirection();

app.MapGet("/", async (IAuthenticationSchemeProvider authenticationSchemeProvider) =>
    (await authenticationSchemeProvider.GetDefaultAuthenticateSchemeAsync())?.Name ?? "[null]");

app.Run();


internal class DisableAutoDefaultSchemeAuthenticationSchemeWrapper : IAuthenticationSchemeProvider
{
    private readonly AuthenticationSchemeProvider _authenticationSchemeProvider;
    private readonly AuthenticationOptions _options;

    public DisableAutoDefaultSchemeAuthenticationSchemeWrapper(IOptions<AuthenticationOptions> authnOptions)
    {
        _authenticationSchemeProvider = new(authnOptions);
        _options = authnOptions.Value;
    }

    public void AddScheme(AuthenticationScheme scheme) => _authenticationSchemeProvider.AddScheme(scheme);

    public Task<IEnumerable<AuthenticationScheme>> GetAllSchemesAsync() => _authenticationSchemeProvider.GetAllSchemesAsync();

    public Task<AuthenticationScheme?> GetDefaultAuthenticateSchemeAsync() =>
        _options.DefaultAuthenticateScheme is not null
            ? GetSchemeAsync(_options.DefaultAuthenticateScheme)
            : GetDefaultSchemeAsync();

    public Task<AuthenticationScheme?> GetDefaultChallengeSchemeAsync() =>
        _options.DefaultAuthenticateScheme is not null
            ? GetSchemeAsync(_options.DefaultAuthenticateScheme)
            : GetDefaultSchemeAsync();

    public Task<AuthenticationScheme?> GetDefaultForbidSchemeAsync() =>
        _options.DefaultForbidScheme is not null
            ? GetSchemeAsync(_options.DefaultForbidScheme)
            : GetDefaultSchemeAsync();

    public Task<AuthenticationScheme?> GetDefaultSignInSchemeAsync() =>
        _options.DefaultSignInScheme is not null
            ? GetSchemeAsync(_options.DefaultSignInScheme)
            : GetDefaultSchemeAsync();

    public Task<AuthenticationScheme?> GetDefaultSignOutSchemeAsync() =>
        _options.DefaultSignOutScheme is not null
            ? GetSchemeAsync(_options.DefaultSignOutScheme)
            : GetDefaultSchemeAsync();

    public Task<IEnumerable<AuthenticationScheme>> GetRequestHandlerSchemesAsync() =>
        _authenticationSchemeProvider.GetRequestHandlerSchemesAsync();

    public Task<AuthenticationScheme?> GetSchemeAsync(string name) =>
        _authenticationSchemeProvider.GetSchemeAsync(name);

    public void RemoveScheme(string name) => _authenticationSchemeProvider.RemoveScheme(name);

    private Task<AuthenticationScheme?> GetDefaultSchemeAsync()
       => _options.DefaultScheme != null
           ? GetSchemeAsync(_options.DefaultScheme)
           : Task.FromResult<AuthenticationScheme?>(null);
}