Enable gradient when alias is cleared

Marlin/src/feature/mixing.h

     // Refresh the gradient after a change
     static void refresh_gradient() {
       #if ENABLED(GRADIENT_VTOOL)
-        const bool is_grd = (selected_vtool == gradient.vtool_index);
+        const bool is_grd = (selected_vtool == gradient.vtool_index || gradient.vtool_index == -1);
       #else
         constexpr bool is_grd = true;
       #endif

Marlin/src/gcode/feature/mixing/M166.cpp

 
   if (parser.seen('S')) mixer.gradient.enabled = parser.value_bool();
 
+  mixer.refresh_gradient();
+
   SERIAL_ECHOPGM("Gradient Mix ");
   serialprint_onoff(mixer.gradient.enabled);
-
   if (mixer.gradient.enabled) {
 
-    mixer.refresh_gradient();
-
     #if ENABLED(GRADIENT_VTOOL)
       if (mixer.gradient.vtool_index >= 0) {
         SERIAL_ECHOPAIR(" (T", int(mixer.gradient.vtool_index));