Well, it was some kind of a bug that, to the best of my knowledge, still existed in version 1.3, so it got never fixed. Given that there's still a debate whether 1.3 is an actual improvement over 1.2, ...
there was never a 1.3 patch only 1.2 and in my experience of both 1.1. and 1.2 no one could fire through forests. - those were solid blocks - i have hid alongside them many a time to keep out of view of enemy fire, or what in ambush for the approaching column.
what the AI could see through however, were the thin tree lines - these sprites are merely decorative and really just exist to make it harder for anyone doing manual gunnery.
in my experience of both 1.1. and 1.2 no one could fire through forests.
No human player, yes. Not so computer-controlled units. I cannot rule out that this would manifest only under selected circumstances, but it happened to me several times.
If I remember correctly the last patch that was supposed to bug fix outstanding issues Was cancelled in favour of a new release that never materialised. It still brings a tear to my eye.LoL
