The curve disappears as soon as I move any control point.
I checked the same file in Rhino 8, and this issue does not occur there. The curve remains visible when I move the control points. It seems to be specific to Rhino 9 WIP.
bug curva visualizzazione.3dm (344.3 KB)
I think I may have found the issue: Soft Transform was enabled. When I disable it, the problem disappears. @Joshua_Kennedy