HTTPS Load Balancer Proxy works great with Managed Instance Group but not with unmanaged instance group. We have added few Unmanaged Instance Group to the backend and have instructed Proxy to direct specific traffic to unmanaged group e.g. https://test.example.com to unmanaged instance group. When the testing is done we can stop the instances in unmanaged instance groups. However stopping individual VM instances with in managed group is not possible.
Every thing is working as expected. However, browser takes 10-15 seconds (not always but mostly) to display the page and randomly receives 500 error. It seems that instances in unmanaged group are stopped or Load Balancer does some house keeping which takes long to respond.
Any help or suggestions to fix the response time would be highly appreciated. Direct accessing the web server by avoiding the load balancer works as expected but https can't be used as only Proxy Server has the SSL certificate.
I'm taking an educated guess here based on your detailed description of symptoms. As you noticed there's something going on "behind the scenes" of the load balancer and either health checks are failing or some other feature that is responsible for "updating" load balancer that test backend is shut off.
This shouldn't be happening and it looks like a bug.
At this poing I think the best way for you is report a new issue at Google's Issuetracker and include detailed description of what happens. You may link to this question too :)