OpenTelemetry: ActivitySource.StartActivity returns null activity when there are listeners hooked

I use:

OpenTelemetry 1.4.0
OpenTelemetry.Extensions.Hosting 1.4.0
OpenTelemetry.Instrumentation.AspNetCore 1.0.0-rc9.14
and runtime version: .NET 6.0 ASP.NET WebApi using docker base image 6.0-alpine3.17

I have 2 services A and B. Service A exposes REST API endpoint and also service B exposes REST API endpoint.

OpentTelemetry in service A is registered like this:

public static class OTRegistration
{
	private static readonly ActivitySource _activitySource = new ActivitySource(Assembly.GetExecutingAssembly().GetName().Name!, "1.0.0");

	public static void AddOT(this IServiceCollection services)
	{
		services.AddOpenTelemetry()
			.WithTracing(tracerProviderBuilder =>
				tracerProviderBuilder
				.AddSource(_activitySource.Name)
				.ConfigureResource(resource => resource.AddService(_activitySource.Name))
				.AddAspNetCoreInstrumentation()
				);
	}
}

A client calls service A endpoint and next this endpoint calls service B. In a call to service B service A randomly sends traceparent header which ends on sometimes 00 and sometimes 01, for example: 00-000000000000000056473954588e71ac-36ae11cc57b1e9c1-00.
When it ends on 00 service B creates null activities:

Activity? activity = _activitySource.StartActivity("TestActivity");

I double checked that in service B is hooked listener when the activity is created. I added some logs to prove it:

Activity was created as null. | OperationName=file.uploaded, traceparent=00-000000000000000056473954588e71ac-36ae11cc57b1e9c1-00, HasListeners=True.

Activity was created as not null. | OperationName=file.uploaded, traceparent=00-0000000000000000517d22c0e0b634be-5b066682501cc9b8-01, HasListeners=True, RootId=0000000000000000517d22c0e0b634be, ParentId=00-0000000000000000517d22c0e0b634be-a258d1b5dddfc7de-01, Id=00-0000000000000000517d22c0e0b634be-58691a9e53e3ae37-01.

I see that https://learn.microsoft.com/en-us/dotnet/api/system.diagnostics.activitytraceflags?view=net-6.0 has to values None (0) and Recorded (1) so that`s why I suspect that 00 flag means that activity will not be created.

Any ideas why service A sometimes ends the traceparent with 00 and sometimes with 01?
How to control this behavior to not have any randomness?

The last part of traceparent determines if activity should be sampled or no: https://www.w3.org/TR/trace-context/#trace-flags

Sampler is responsible to set/determine if the activity should be recorder/created.

By default Sample is set to parentbased_always_on. It means that if the parent was not recorder, then the there is no reason to create new acitivity.

You can overwrite this functionality by call .SetSampler(new AlwaysOnSampler()) in Service B or check why Service A decided not to record activity (maybe it is propagated from the client of Service A?).