There is no point in providing map seeds as this has happened on over 5 different run throughs of this particular skirmish option today.
In unlikely allies it states “You and your ally start together, separated by a thick line of tress from the other alliances. Chop through the tree line to get to your opponents.”
On the map there is no separating tree line spawning. Since this is normal for Black Forest to not have that separating tree line I was expecting some specific seed or some other mechanic but in all cases when I explore the map there is always a route to the other alliances.