diff --git a/src/slic3r/GUI/GLCanvas3D.cpp b/src/slic3r/GUI/GLCanvas3D.cpp
index 91b4a2855..2d57d5eb7 100644
--- a/src/slic3r/GUI/GLCanvas3D.cpp
+++ b/src/slic3r/GUI/GLCanvas3D.cpp
@@ -1700,8 +1700,10 @@ void GLCanvas3D::Selection::flattening_rotate(const Vec3d& normal)
     }
 
 #if !DISABLE_INSTANCES_SYNCH
+    // we want to synchronize z-rotation as well, otherwise the flattening behaves funny
+    // when applied on one of several identical instances
     if (m_mode == Instance)
-        _synchronize_unselected_instances();
+        _synchronize_unselected_instances(true);
 #endif // !DISABLE_INSTANCES_SYNCH
 
     m_bounding_box_dirty = true;
@@ -1911,7 +1913,6 @@ void GLCanvas3D::Selection::erase()
         {
             items.emplace_back(ItemType::itInstance, i.first, i.second);
         }
-
         wxGetApp().obj_list()->delete_from_model_and_list(items);
     }
     else if (is_mixed())
@@ -2573,7 +2574,7 @@ void GLCanvas3D::Selection::_render_sidebar_size_hint(Axis axis, double length)
 }
 #endif // ENABLE_SIDEBAR_VISUAL_HINTS
 
-void GLCanvas3D::Selection::_synchronize_unselected_instances()
+void GLCanvas3D::Selection::_synchronize_unselected_instances(bool including_z)
 {
     std::set<unsigned int> done;  // prevent processing volumes twice
     done.insert(m_list.begin(), m_list.end());
@@ -2606,7 +2607,7 @@ void GLCanvas3D::Selection::_synchronize_unselected_instances()
             if ((v->object_idx() != object_idx) || (v->instance_idx() == instance_idx))
                 continue;
 
-            v->set_instance_rotation(Vec3d(rotation(0), rotation(1), v->get_instance_rotation()(2)));
+            v->set_instance_rotation(Vec3d(rotation(0), rotation(1), including_z ? rotation(2) : v->get_instance_rotation()(2)));
             v->set_instance_scaling_factor(scaling_factor);
             v->set_instance_mirror(mirror);
 
diff --git a/src/slic3r/GUI/GLCanvas3D.hpp b/src/slic3r/GUI/GLCanvas3D.hpp
index c2f2074c1..63a4edbef 100644
--- a/src/slic3r/GUI/GLCanvas3D.hpp
+++ b/src/slic3r/GUI/GLCanvas3D.hpp
@@ -597,7 +597,7 @@ public:
         void _render_sidebar_scale_hint(Axis axis) const;
         void _render_sidebar_size_hint(Axis axis, double length) const;
 #endif // ENABLE_SIDEBAR_VISUAL_HINTS
-        void _synchronize_unselected_instances();
+        void _synchronize_unselected_instances(bool including_z = false);
         void _synchronize_unselected_volumes();
 #if ENABLE_ENSURE_ON_BED_WHILE_SCALING
         void _ensure_on_bed();