Help me check why i have this triangles on my mesh

why my terrain have this triangles when i upload the mesh into blender, is it caused by bad uv map??, i checked uv map look fine since the texture is attaching correctly to the mesh ? on blender it look ok but when loading on trhee.js i have this error, also remade normal and tried several export options but still the same error

i tried the same mesh on here https://gltf-viewer.donmccurdy.com/ and it load correctly witht those ugly triangles, on code im jsut doign the basic code

this.model = model.scene.clone(true);
this.root.add(this.model);

the problem is when i enable cast shadow on the terrain, terain should only recieve shadows , forget about it ,had to compare wih https://gltf-viewer.donmccurdy.com/