The fact that snmpwalk
returns an OID that you consequently cannot snmpget
indicates a bug in the SNMP agent of the device you are accessing.
As frustrating as this is it isn't uncommon, even in enterprise equipment. I've encountered SNMP agent bugs in Cisco and Nortel telecommunications equipment costing hundreds of thousands of dollars.
You do have a work-around. Consider writing a BASH script to snmpwalk
the desired value and scrape the result. And tell Cacti to use your custom script instead of accessing via SNMP directly.