A reload of the varnish server does not destroy your cache but reloads your vcl. Check your vcl file for errors before trying a reload.
varnishd -C -f /etc/varnish/default.vcl && systemctl reload varnish.service && systemctl status varnish.service varnishncsa.service
Default in varnish 5.2.1 keepalive is enabled, and set with a timeout of timeout_idle=5 seconds. which means that if no traffic goes through the tcp pipe in 5 seconds the TCP connection is closed.
If you are running HLS with a chunksize of 8 seconds you want to change timeout_idle to 8 seconds. you can change timeout_idle in a default installation by adding the following to /etc/varnish/varnish.params.
DAEMON_OPTS="-p timeout_idle=8"