I'm having the same issue on Cisco ASA version 9.15(1).
From time to time our SNMP monitoring software just gives error because the ASA doesn't return the full list of active VPN tunnels.
I can reproduce this behavior using snmpwalk on a Linux machine.
Has anyone been able to find a workaround?
The OID I use is "1.3.6.1.4.1.9.9.171.1.2.3.1.7"