Fix G34 move in wrong direction (#18188)

Marlin/src/gcode/calibrate/G34_M422.cpp

@@ -294,11 +294,14 @@ void GcodeSuite::G34() {
           // Check for less accuracy compared to last move
           if (last_z_align_move[zstepper] < z_align_abs * 0.7f) {
             SERIAL_ECHOLNPGM("Decreasing accuracy detected.");
+            if (DEBUGGING(LEVELING)) DEBUG_ECHOLNPAIR("> Z", int(zstepper + 1), " last_z_align_move = ", last_z_align_move[zstepper]);
+            if (DEBUGGING(LEVELING)) DEBUG_ECHOLNPAIR("> Z", int(zstepper + 1), " z_align_abs = ", z_align_abs);
             adjustment_reverse = !adjustment_reverse;
           }
 
-          // Remember the alignment for the next iteration
+          // Remember the alignment for the next iteration, but only if steppers move,
-          last_z_align_move[zstepper] = z_align_abs;
+          // otherwise it would be just zero (in case this stepper was at z_measured_min already)
+          if (z_align_abs > 0) last_z_align_move[zstepper] = z_align_abs;
         #endif
 
         // Stop early if all measured points achieve accuracy target
@@ -312,8 +315,10 @@ void GcodeSuite::G34() {
         #if DISABLED(Z_STEPPER_ALIGN_KNOWN_STEPPER_POSITIONS)
           // Decreasing accuracy was detected so move was inverted.
           // Will match reversed Z steppers on dual steppers. Triple will need more work to map.
-          if (adjustment_reverse)
+          if (adjustment_reverse) {
             z_align_move = -z_align_move;
+            if (DEBUGGING(LEVELING)) DEBUG_ECHOLNPAIR("> Z", int(zstepper + 1), " correction reversed to ", z_align_move);
+          }
         #endif
 
         // Do a move to correct part of the misalignment for the current stepper