I got the same problem but with a different version.
just to test I would develop a simple check just to get an OID and see if it appears on cmk, doing a snmpwalk from a host and send the piggyback to the fortigate.
What it seems to me is that may be happening some mismatch between plugin version and fortunate fortigate (damn autocorrect) version.
It happened once to me, so the approach for me was local checks/piggyback.
Cheers