After the connection is closed, your app should open a new connection using resume_gateway_url rather than the URL used to initially connect. If your app doesn't use the resume_gateway_url when reconnecting, it will experience disconnects at a higher rate than normal.
As per Discord's Gateway API documentation
After the connection is closed, your app should open a new connection using resume_gateway_url rather than the URL used to initially connect. If your app doesn't use the resume_gateway_url when reconnecting, it will experience disconnects at a higher rate than normal.