When Zabbix issues an snmpbulk request, the device returns many responses in one packet. If any of the responses are NOSUCHOBJECT or NOSUCHINSTANCE zabbix stops processing the response and ignores all subsequent values, which are still valid responses to other items in the bulk request. Apparently some higher level assumes these other values have been processed as occasionally incorrect data is logged for other items that were in the packet request, but were after an item that returned NOSUCHOBJECT/INSTANCE.

Disabling bulk request caused the anomalous values to stop appearing.

Looking through the code zabbix_server/poller/checks_snmp.c it does appear that it stops processing a response at the first NOSUCHOBJECT or NOSUCHINSTANCE it encounters, even though these responses apply to specific OIDs in the request, and not to the entire request.

details here:

https://www.zabbix.com/forum/showthread.php?t=50605