Have the mibs changed with TrueNAS 12?
zpoolReadOps1sec OBJECT-TYPE
    SYNTAX Counter64
    MAX-ACCESS read-only
    STATUS current
    DESCRIPTION
        "The number of read I/O operations sent to the pool or device,
        including metadata requests (over 1 second interval)."
    ::= { zpoolEntry 12 }
zpoolWriteOps1sec OBJECT-TYPE
    SYNTAX Counter64
    MAX-ACCESS read-only
    STATUS current
    DESCRIPTION
        "The number of write I/O operations sent to the pool or device
        (over 1 second interval)."
    ::= { zpoolEntry 13 }
zfsArcSize OBJECT-TYPE
    SYNTAX Gauge32
    MAX-ACCESS read-only
    STATUS     current
    DESCRIPTION
        ""
    ::= { arc 1 }
zfsL2ArcSize OBJECT-TYPE
    SYNTAX     Gauge32
    MAX-ACCESS read-only
    STATUS     current
    DESCRIPTION
        ""
    ::= { l2arc 5 }diff output between the 12.0-BETA and 11.3-U3.2 FREENAS-MIB.txts shows the changes are mostly to MIBs for device temps:10c10
<     LAST-UPDATED "202005280000Z"
---
>     LAST-UPDATED "201905120000Z"
19c19
<     REVISION     "202005280000Z"
---
>     REVISION     "201905120000Z"
327,328c327
<         zvolAvailable                Integer32,
<         zvolReferenced               Integer32
---
>         zvolAvailable                Integer32
380,387d378
< zvolReferenced OBJECT-TYPE
<     SYNTAX     Integer32 (0..2147483647)
<     MAX-ACCESS read-only
<     STATUS     current
<     DESCRIPTION
<         ""
<     ::= { zvolEntry 7 }
<
573,619d563
< hddTempTable OBJECT-TYPE
<     SYNTAX      SEQUENCE OF HDDTempEntry
<     MAX-ACCESS  not-accessible
<     STATUS      current
<     DESCRIPTION
<         "Table of HDDs and their temperatures."
<     ::= { freeNas 3 }
<
< hddTempEntry OBJECT-TYPE
<     SYNTAX      HDDTempEntry
<     MAX-ACCESS  not-accessible
<     STATUS      current
<     DESCRIPTION
<         "An entry containing a HDD and its temperature."
<     INDEX       { hddTempIndex }
<     ::= { hddTempTable 1 }
<
< HDDTempEntry ::= SEQUENCE {
<     hddTempIndex    Integer32,
<     hddTempDevice   DisplayString,
<     hddTempValue    Gauge32
< }
<
< hddTempIndex OBJECT-TYPE
<     SYNTAX      Integer32 (0..65535)
<     MAX-ACCESS  read-only
<     STATUS      current
<     DESCRIPTION
<         "Reference index for each observed HDD."
<     ::= { hddTempEntry 1 }
<
< hddTempDevice OBJECT-TYPE
<     SYNTAX      DisplayString
<     MAX-ACCESS  read-only
<     STATUS      current
<     DESCRIPTION
<         "The name of the HDD we are reading temperature from."
<     ::= { hddTempEntry 2 }
<
< hddTempValue OBJECT-TYPE
<     SYNTAX      Gauge32
<     MAX-ACCESS  read-only
<     STATUS      current
<     DESCRIPTION
<         "The temperature of this HDD in mC."
<     ::= { hddTempEntry 3 }
<
[root@zabbix ~]# snmptranslate -On FREENAS-MIB::zfsArcC .1.3.6.1.4.1.50536.1.4.6
[root@zabbix ~]# snmpwalk -v 2c -c public 192.168.40.40 FREENAS-MIB::zfsArcC FREENAS-MIB::zfsArcC = No Such Object available on this agent at this OID
I built a complete new Zabbix VMTry removing and reimporting the FREENAS-MIB.
[root@zabbix ~]# snmpwalk -v 2c -c public 192.168.40.40 IF-MIB::ifDescr IF-MIB::ifDescr.1 = STRING: ix0 IF-MIB::ifDescr.2 = STRING: ix1 IF-MIB::ifDescr.3 = STRING: bge0 IF-MIB::ifDescr.4 = STRING: bge1 IF-MIB::ifDescr.5 = STRING: lo0 IF-MIB::ifDescr.6 = STRING: pflog0 IF-MIB::ifDescr.7 = STRING: lagg0 IF-MIB::ifDescr.8 = STRING: vlan10 IF-MIB::ifDescr.9 = STRING: vlan40 IF-MIB::ifDescr.10 = STRING: vlan60 IF-MIB::ifDescr.11 = STRING: vlan110 IF-MIB::ifDescr.12 = STRING: vlan120 IF-MIB::ifDescr.13 = STRING: ue0
[root@zabbix mibs]# snmpwalk -v 2c -c public 192.168.40.40 FREENAS-MIB::zfsArcSize FREENAS-MIB::zfsArcSize = No Such Object available on this agent at this OID