GCP: How to check the log for changes in the health state of a load balancer backend

I'm using GCP's an unmanaged HTTP external load balacer, and I have several Nginx servers running on its backend.

I wanted to check the logs for changes in the health status of the Nginx servers, but could not find a way to do so.

First, I enabled logging for the health check assigned to the backend service. Then, I stopped the service of one Nginx server to test it. Then I selected GCE Health Check from the Resource Type in the Logs Explorer, but only logs related to the creation, update, and deletion of the health check itself appeared.

Next, I enabled logging for the backend service and did the same experiment. However, similarly, only logs related to the creation, update, and deletion of the backend service itself appeared.

I have three questions:

1. Doesn't the logging of the health check log the changes in the health status of the monitored service?
2. Doesn't the logging of backend services log the health status changes of the instances belonging to the backend?
3. How can I check when each Nginx server became unhealthy and when it became healthy in the log?