Broken surface caused by trimming multiple regions

I was attempting to create model of terrain by cutting out multiple regions defined by planar polylines. After some manipulation with terrain, the isocurves and edges looked fine, but the “fill” of the surface has fallen apart to these jagged shapes.

The cut-outs are made from rather complex planar polylines (I guess this is where the problem started). Rhino does not consider the surface as a “bad object”, I have tried rebuilding it, creating a mesh from it (attached image), saving it “as small”, moving to smaller coordinates, and doing the same on another PC (with Rhino 7), but without any success. Untrimming all edges “fixes” the surface, untrimming only some of them unfortunately does not.

I would be grateful for any advice on how to fix or work with this surface. I am also aware that this is not the optimal way how to create terrain models, any tips for a better approach to modelling would be highly appreciated, I am thinking about trying the Lands Design plugin. I have attached the surface model below.
Thank you very much in advance.
BrokenSurface.3dm (3.4 MB)

Can’t download your file here ( on a phone) but I suspect the model is very far from the origin. If you move the model closer to the origin, select all and run RefreshShade, does it look better?

You will probably need to redo the mesh from NURBS part.

Thank you for your reply. Unfortunately, that did not solve the problem. I have previously tried moving the surface to the origin based on advice from similar topics (posted screenshots are with the surface centred on the origin, size of the surface of about 120 units - meters). RefreshShade did not make any difference.