Azure Function Configuration values not available in function attributes

I have an Azure Function running in Azure Portal. Its configuration resides in a configuration resource, some values are in a key vault resource. The Azure Function was implemented in .NET 6 with the in-process model. I migrated the code to .NET 8 using the isolated-worker model.

Here's my Program.cs code:

using System.Reflection;
using Azure.Identity;
using Microsoft.Azure.Functions.Worker;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;

namespace My.AzureFunctions
{
    public class Program
    {
        public static void Main()
        {
            var host = new HostBuilder()
                .ConfigureFunctionsWebApplication()
                .ConfigureAppConfiguration(config =>
                {
                    var builtConfig = config.Build();
                    var appConfigurationName = builtConfig
                    
                    var defaultAzureCredentialOptions = new DefaultAzureCredentialOptions
                    {
                        ExcludeAzureCliCredential = false,
                        ExcludeAzurePowerShellCredential = true,
                        ExcludeEnvironmentCredential = false,
                        ExcludeManagedIdentityCredential = false,
                        ExcludeVisualStudioCodeCredential = true,
                        ExcludeVisualStudioCredential = true
                    };
                    config.AddAzureAppConfiguration(options =>
                    {
                        options.Connect(new Uri($"https://{appConfigurationName}.azconfig.io"),
                        new DefaultAzureCredential(defaultAzureCredentialOptions))
                            .ConfigureKeyVault(kvOptions =>
                            {
                                kvOptions.SetCredential(
                                    new DefaultAzureCredential(defaultAzureCredentialOptions));
                                kvOptions.SetSecretRefreshInterval(TimeSpan.FromHours(1));
                            });
                    });
                    config.AddEnvironmentVariables();

                    foreach (var kvp in config.Build().AsEnumerable())
                    {
                        if (!string.IsNullOrEmpty(kvp.Value))
                        {
                            Environment.SetEnvironmentVariable(kvp.Key, kvp.Value);
                        }
                    }
                })
                .ConfigureServices((context, services) =>
                {
                    // Bind configurations to options
                    // ...
                })
                .Build();

            host.Run();
        }
    }
}

The Functions look like this:

    [Function("TestTimerTrigger")]
    public void TestTimerTrigger(
        // This causes an error
        [TimerTrigger("%TimeValue%)] TimerInfo timerInfo,
        FunctionContext functionContext,
        CancellationToken cancellationToken)
    {
        // Here TimeValue residing in Azure Configuration can be read.
        _logger.LogInformation("TestTimerTrigger function executed with: {TimeValue}", _configuration["TimeValue"]);

        // Also accessible here
        _logger.LogInformation("TestTimerTrigger function executed with: {TimeValue}", Environment.GetEnvironmentVariable("TimeValue"));
    }

    [Function("MyEventTrigger")]
    // The value cannot be read here, either.
    public async Task Run([EventHubTrigger("", Connection = "MyEventHub:ConnectionString")] EventData[] messages, 
        FunctionContext functionContext)
    {
        // ...
    }

I get these errors:

The 'TestTimerTrigger' function is in error: Microsoft.Azure.WebJobs.Host: Error indexing method 'Functions.TestTimerTrigger'. Microsoft.Azure.WebJobs.Host: '%TimeValue%' does not resolve to a value.

Microsoft.Azure.WebJobs.Extensions.EventHubs: EventHub account connection string with name 'MyEventHub:ConnectionString' does not exist in the settings. Make sure that it is a defined App Setting.

In the old .NET 6 in-process model Function I could access the configuration values in the Function Attributes, but after changing the code to .NET 8 and the isolated worker model, I have the described problem. The configuration itself and the way to access it has not changed. What am I doing wrong?