I fixed the problem by adding the AzureWebAppDiagnostics Logging provider.
A few months ago this was not needed, so it seems the defaults have been changed at some time.
public static IHostBuilder CreateHostBuilder(string[] args) =>
Host.CreateDefaultBuilder(args)
.ConfigureLogging(logging =>
{
logging.AddAzureWebAppDiagnostics();
})
.ConfigureWebHostDefaults(webBuilder =>
{
webBuilder.UseStartup();
});
See https://video2.skills-academy.com/en-us/aspnet/core/fundamentals/logging/?view=aspnetcore-3.1