diff --git a/src/bonsai/bonsai/bim/module/model/wall.py b/src/bonsai/bonsai/bim/module/model/wall.py
index bc54335c91..428c999274 100644
--- a/src/bonsai/bonsai/bim/module/model/wall.py
+++ b/src/bonsai/bonsai/bim/module/model/wall.py
@@ -1619,13 +1619,16 @@ class DumbWallJoiner:
             r.RelatedOpeningElement for r in list(element1.HasOpenings) if r.RelatedOpeningElement.HasFillings
         ]:
             rel = opening.HasFillings[0]
-            filling = rel.RelatedBuildingElement
-            filling_obj = tool.Ifc.get_object(filling)
-            filling_location = filling_obj.matrix_world.translation
-            _, filling_position = mathutils.geometry.intersect_point_line(filling_location.to_2d(), *axis_world_2d)
             min_t, max_t = _opening_axis_extent(opening, axis_world_2d, unit_scale)
+            # Use the opening's axis-projected midpoint to classify the side.
+            # The filling's ``matrix_world.translation`` is flip-fragile —
+            # ``flip_object`` rotates 180° + translates so the bbox stays
+            # visually in place, moving the door origin to the opposite
+            # corner, which would mis-classify a flipped door centred over
+            # the cut.
+            opening_midpoint = (min_t + max_t) / 2
             void_straddles = min_t < cut_percentage < max_t
-            if filling_position > cut_percentage:
+            if opening_midpoint > cut_percentage:
                 # The filling should be moved from element1 to element2.
                 new_opening = ifcopenshell.api.root.copy_class(tool.Ifc.get(), product=opening)
                 new_opening.VoidsElements[0].RelatingBuildingElement = element2
@@ -1640,13 +1643,16 @@ class DumbWallJoiner:
 
                 rel.RelatingOpeningElement = new_opening
 
-                # Remove the old opening
-                ifcopenshell.api.feature.remove_feature(tool.Ifc.get(), feature=opening)
-
                 if void_straddles:
                     # Filling moved to element2, but void straddles — add a
-                    # pure-void copy back to element1 so its body still gets cut.
-                    _add_void_copy(element1, new_opening)
+                    # pure-void copy back to element1. Read from the original
+                    # ``opening`` whose ObjectPlacement still references
+                    # element1; ``new_opening`` was rebound to element2 and
+                    # would copy element2's frame instead.
+                    _add_void_copy(element1, opening)
+
+                # Remove the old opening
+                ifcopenshell.api.feature.remove_feature(tool.Ifc.get(), feature=opening)
             elif void_straddles:
                 # Filling stays on element1, but void straddles — add a pure-void
                 # copy to element2 so its body gets cut.
diff --git a/src/bonsai/test/bim/module/model/test_wall_split_filled_opening.py b/src/bonsai/test/bim/module/model/test_wall_split_filled_opening.py
new file mode 100644
index 0000000000..9bbfcbb88a
--- /dev/null
+++ b/src/bonsai/test/bim/module/model/test_wall_split_filled_opening.py
@@ -0,0 +1,66 @@
+# Bonsai - OpenBIM Blender Add-on
+# Copyright (C) 2026
+#
+# This file is part of Bonsai.
+#
+# Bonsai is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# Bonsai is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with Bonsai.  If not, see <http://www.gnu.org/licenses/>.
+#
+# This file was generated with the assistance of an AI coding tool.
+
+"""Pins two contracts in ``DumbWallJoiner.split``'s filled-opening branch:
+
+1. Side classification reads the opening's axis-projected midpoint, not
+   the filling's ``matrix_world.translation``. The filling origin is
+   flip-fragile — ``flip_object`` rotates the filler 180° + translates so
+   the bbox stays visually in place, which would mis-classify a flipped
+   door centred over the cut.
+2. When the void straddles the cut and the filling moves to element2,
+   the void copy for element1 is taken from the ORIGINAL opening (whose
+   ``ObjectPlacement`` still references element1), not the rebound
+   ``new_opening`` (whose ``PlacementRelTo`` was swapped to element2)."""
+
+import inspect
+
+import pytest
+
+pytestmark = pytest.mark.wall
+
+
+def _split_source():
+    from bonsai.bim.module.model.wall import DumbWallJoiner
+
+    return inspect.getsource(DumbWallJoiner.split)
+
+
+def test_side_classification_uses_opening_midpoint_not_filling_origin():
+    source = _split_source()
+    assert "opening_midpoint" in source
+    # The pre-fix code projected the filling's world translation onto the
+    # axis to classify; that path must be gone.
+    assert "filling_obj.matrix_world.translation" not in source
+
+
+def test_void_copy_reads_from_original_opening_before_remove():
+    source = _split_source()
+    # Locate the "filling moves to element2" branch via the opening
+    # midpoint check; the void-copy and the trailing remove_feature both
+    # live inside this branch, after the prior unfilled-opening loops.
+    branch_start = source.index("if opening_midpoint > cut_percentage:")
+    branch = source[branch_start:]
+    add_idx = branch.index("_add_void_copy(element1, opening)")
+    remove_idx = branch.index("feature.remove_feature(tool.Ifc.get(), feature=opening)")
+    # Read-from-original is the whole point — the rebound ``new_opening``
+    # references element2's frame and would shift the void to element1's
+    # origin in element2's local coords.
+    assert add_idx < remove_idx