I think there are two bugs in the fog settings.
It's pretty confusing.
<Bug 1>
When playing a miz.file with fog set to auto, you can't get LA for IR missiles.
<Bug 2>
Once you play/replay a miz/trk.file with fog set to auto, other miz/trk.files seem to be also affected. You can't change the fog settings unless restarting DCS.
I've attached 3 trk.files below. They all have the same content, but I've only changed the fog settings.
[Su27 fog off], [Su27 fog auto], [Su27 fog manual (thickness:0)]
In my PC environment, I could get LA with [fog off] and [fog manual (thickness:0)], but not with [fog auto].
And if I replay [fog auto] first, I could not get LA even with [fog off] and [fog manual (thickness:0)].
Su27 fog off.trk
Su27 fog auto.trk
Su27 fog manual.trk