Here’s the same with runway profile, lowered by 40m for better visibility of the issues at hand:
TIN active:
TIN not active:
Also tried to place a terraforming polygon flattening everything with a lower and higher priority. Higher priority works as expected and flattens everything including the TIN. Lower priority doesn’t make a difference. We really need the ability to create heightmaps without it using the TIN grid as a reference after building.