Comparing with your file, I finally found that I did a mistake: I didn’t check “Surfaces” in the drop-down. It would then select the sphere only when clicking on the isocurves.
Sorry for that!
Still wierd that a sphere was selected by its isocurves though… maybe because a surface is considered as a brep, but only when selected from its edges/isocurves… or maybe i miss something else…
Anyway, I will check “Surfaces” in the menu now.