XY plane intersection give non planar curve?

I am pretty sure this is a tolerance issue but I was not able to solve it.

I am cutting a surface with XY planes and the resulting curves can’t be offseted because they are not planar?

Planes Cut.gh (9.2 KB)

I opened your file and do not see the error.

Rhino 8 SR4 2024-2-6 (Rhino 8, 8.4.24037.1500)

File units: Millimeters

Absolute tolerance: 0.001

no errors also on R7.37.24004.15001, 2024-01-04
units mm tol 0.001