I've never heard of anyone tree surfing around Valeia!
I suspect that you did encounter invisible walls/boundaries. Most often when someone is trying tree surfing they're doing it in the Brimlock area and you sort of know that there is a barrier to the south and that you want to head west into the Lost Sea. I wonder if you will inspire others to go tree surfing in unexpected areas!
If a monster spawns in the forest and there is no character there to encounter them, do they exist?
I think monsters spawn in relation ship to your party.
Wyv