Fix camera projection matrix

Hey. I make an analogue of the game Portal. In general, drawing portals works correctly. But, when the player’s camera is very close to the portal, something like Z-fighting occurs. How can this be fixed? For several days I can’t solve this. Watch it on youtube - https://youtu.be/tVYeXTFM8Jk

I set the projection matrix by code:

 Vector3 pos = targetPortal.transform.position; // position of another portal plane
 Vector3 normal = targetPortal.transform.forward; // normal of another portal plane
 Vector4 clipPlane = PortalUtils.CameraSpacePlane(Cams[i], pos, normal, ClipPlaneOffset, 1.0f);
 
 Matrix4x4 projection = CameraDefault.CalculateObliqueMatrix(clipPlane);
 Cams[i].projectionMatrix = projection;

CameraSpacePlane metod code:

 public static Vector4 CameraSpacePlane(Camera cam, Vector3 pos, Vector3 normal, float ClipPlaneOffset, float sideSign)
         {
             Vector3 offsetPos = pos + (normal * ClipPlaneOffset);
             Matrix4x4 m = cam.worldToCameraMatrix;
             Vector3 cpos = m.MultiplyPoint(offsetPos);
             Vector3 cnormal = m.MultiplyVector(-normal) * sideSign;
             return new Vector4(cnormal.x, cnormal.y, cnormal.z, Vector3.Dot(-cpos, cnormal));
         }

Help pls =)