I'm not sure if this is the right way to fix it. Not sure what ACTIVE really means here.
The same issue might exist for some other properties as well.

For fuel it seems to be ok to always have it active if mfs->type == FLUID_FLOW_TYPE_FIRE || mfs->type == FLUID_FLOW_TYPE_SMOKEFIRE.

Also I noticed that you implicitly convert float to bool in many if statements. I think it would be better to use != 0.0f instead, but that's a different topic.

Note: Only checking if the fuel is zero or if it animated might also not work if the value is controlled by some python script.