strange that last bastion is all on snow. When I handpicked the values for the latest tree change (the one currently in use) I made sure to keep at least roughly the same old affinity. So all trees that grew, for example, on summer meadow in build 18 will still grow in summer meadow with similar percentages.
It is possible that I made a mistake for one terrain type. It is also possible I did it intentionally: there was a request to make one buildable but barren terrain for every biome, so that one could make port spaces without trees growing spontaneously on them. those barren terrains are barren steppe for summer, desert 4 - and, to a lesser extent, dry soil - on desert, igneous rock on wasteland, and snow on winter. I think a barren buildable terrain is a useful thing to have anyway. Not that it is completely barren; you should be able to get a decent productivity if you place 5 foresters for every woodcutter and give them plenty of space. but yeah, not good.
I agree with the proposal to change the terrain of that map to tundra. I also agree in including some new map in the default maps that come with the game. Now, I'd like to see one of my maps for that, but it would be more fair to vote or something. or at least let choose someone who doesn't have a conflict of interests