dlasher i fixed mine the same way like you, but there is more nasty bug with DUAL GPU adapters. Fans there are -1.
So if you have 6 adapters with IDs 0 1 2 3 4 5 . 1 of them is DUAL GPU. Lets say last one.
0 1 2 3 4 are physical. ID 5 is the second GPU. ID 5 must be ignored for fan polling. For load, temp etc is fine.
I would love to see a dump of this (with
gpu_snmp.pl fan) if you can provide it. Since my rig only has two dual GPUs, I don't really know what this would look like in a mixed system. My dual 6990's report back two fan set points per card even though there is really only one attached fan.