Here's some more information about what changed regarding terraforming with SU12.
A new terraforming technique was developed. Its aim is get rid of the staircase effect we have on slopes.
Basically, pre SU12 terraformers had their own dedicated elevation map with a lower resolution than what the actual terrain allowed. With SU12 it now handles elevation at native terrain resolution so we are no longer limited by the elevation map.
Pictures are worth a thousand words so here's an example of how it improves the end result:
Obviously, as introducing this change can cause regressions on your sceneries, this is versioned and existing packages will still use the old terraforming technique.
However, when opening your source project with the Scenery Editor and saving it, your terraforming shapes will be switched to the new version and new terraforming will apply. This is not configurable.
Some changes can be expected and require modifications on your side.
The changes we had anticipated were for example scenery objects floating above the ground after its shape had changed.
There seems to be an additional case we haven't anticipated and is the result of a misunderstanding of how stacked terraformers work in MSFS.
This is the problem shown here: Terraforming Broken? SU12 - MSFS DevSupport (flightsimulator.com)
When stacking terraforming rectangles, the falloff will only be used for the lowest priority rectangle.
All rectangles on top will have their falloff behave as if it was set to 0 and give the sharpest transition.
As I mentioned earlier, this sharpest transition, given the old elevation map resolution was low, was giving fairly smooth slopes. Smooth enough for some scenery developers to use it as is for smooth areas.
Now as terraforming works at higher resolution, this same sharp transition is way more visible.
We are reviewing what's the best way to address this problem and will come back on the subject later.
Asobo / Sylvain