Geopotential isolines bug likely
A bug? Geopotential isolines do not change when a change the model, from ECMWF to GFS. It seems that ECMWF isolines are displayed whatever model you choose.
You are right. The Geopotential isolines, or isohypses, work well when ECMWF is selected. Switching to GFS, the isolines are still those of ECMWF with the GFS overlay .