Is there a way to add a feature to a terrain model without the whole model retriangulating? ORD CE 2021 R1

Clayton, this subject has come up before on this forum, and there is no way to stop the surface from reprocessing the triangles. That's why some of us will either do any 'flipping' as part of the final surface clean-up. But that's not a solution to additional survey being added a month later. Additionally, some of us basically avoid the flip face tool all together. InRoads never had such a tool so us 'InRoads users" became very accustom to just adding a breakline to force the triangle leg to form the way we want.

Do you use a single terrain model, or is it a complex terrain where you add the individual terrains as they become available?
I assume that when you clean up a single terrain model and add it to a complex terrain, the clean up will maintain. If you later add a new model to the complex terrain, it would be counter intuitive to remove the clean up in the individual terrains that are part of the complex terrain.

It might be worth checking if this is not currently your process.

Adding a new model to a complex terrain still does drop all edits (outside of the border, if you added one). I have tried that before, as well as exporting to a LandXML and re-importing it. Nothing seems to hold the edits.

So let me get this right. I assume these are the steps you take to create the complex terrain:
Edit first terrain
Edit second terrain
Add both terrains to complex terrain
Edit additional terrain
Add additional terrain to complex terrain
Etc.

If you follow these steps the complex terrain will still flip your edits?

As soon as the complex model is created it drops edits. 

Here is an example. The north/south sides of this intersection are two different surfaces. As soon as the complex model is created, they both revert back to the original state. I did it without the border to show the effect more.