There are three instances when a connection could be closed from our server side:
Connection is killed if no data is sent for 15 seconds
When our load balancer restarts its workers. It gives some time for existing connections to finish (approximately 1 hour). Then if there still are live connections, they're killed
The exit node goes offline temporarily
If any of these affect your work flow you can reach out to use and we will see how we can help you.