i have been trying to convert multiple grass areas back to forest for a very long time (months). i have notice a pattern where forest tile will no longer spread at a certain angle from a tree. I know how to fix this manually however this is a bug so it should be fixed.
if you take a look at the picture linked, i have highlighted the areas, these areads will ALWAYS turn to grass, no matter what. they will never turn to forest. I have another area iam trying to teraform and this same patter also occurs. its only had 2 trees though so i thought this picture would be better since its so many.
Around clifs is the same thing, some of the tiles will not convert to forest no matter how many times i reset them,they will always turn to grass.
Its as if the formula always decays the tile based off of the tile diagonal to it. its not random. so if u have a tree with grass or wald under it. it will be impossible to covert that tile to anything other than grass or wald, even if all other sides of that terrian are some other terrain.
