If you run free -h and check Free Memory this should be correct. Your thinking of Available memory which is not reported in LibreNMS as i don’t think it’s available over snmp, you can test yourself against some of the oids http://www.net-snmp.org/docs/mibs/ucdavis.html
Check the discussions here:
#https://github.com/librenms/librenms/issues/5660
#https://github.com/librenms/librenms/issues/3179
Not sure of another way, you could possibly explore a nagios plugin to monitor available memory as a librenms service.