Debug of G2-G3 for UBL (#12386)

Changes only affect UBL.  Everything else is left alone.

Marlin/src/gcode/motion/G2_G3.cpp

@@ -70,6 +70,9 @@ void plan_arc(
   float r_P = -offset[0], r_Q = -offset[1];
 
   const float radius = HYPOT(r_P, r_Q),
+              #if ENABLED(AUTO_BED_LEVELING_UBL)
+                start_L  = current_position[l_axis],
+              #endif
               center_P = current_position[p_axis] - r_P,
               center_Q = current_position[q_axis] - r_Q,
               rt_X = cart[p_axis] - center_P,
@@ -179,7 +182,11 @@ void plan_arc(
     // Update raw location
     raw[p_axis] = center_P + r_P;
     raw[q_axis] = center_Q + r_Q;
-    raw[l_axis] += linear_per_segment;
+    #if ENABLED(AUTO_BED_LEVELING_UBL)
+      raw[l_axis] = start_L;
+    #else
+      raw[l_axis] += linear_per_segment;
+    #endif
     raw[E_AXIS] += extruder_per_segment;
 
     clamp_to_software_endstops(raw);
@@ -198,6 +205,9 @@ void plan_arc(
 
   // Ensure last segment arrives at target location.
   COPY(raw, cart);
+  #if ENABLED(AUTO_BED_LEVELING_UBL)
+    raw[l_axis] = start_L;
+  #endif
 
   #if HAS_LEVELING && !PLANNER_LEVELING
     planner.apply_leveling(raw);
@@ -209,6 +219,9 @@ void plan_arc(
     #endif
   );
 
+  #if ENABLED(AUTO_BED_LEVELING_UBL)
+    raw[l_axis] = start_L;
+  #endif
   COPY(current_position, raw);
 } // plan_arc