PartDesign: adding chamfer renders incorrect result

### Problem description

- created a sketch
- padded the sketch
- added fillets to all (vertical) corners
- added chamfer to top edge
=> top layer rises up and exposes hollow inside of shape/block

### Workbench affected?

Part Design

### Steps to reproduce

- created a sketch
- padded the sketch
- added fillets to all (vertical) corners
- added chamfer to top edge

[paver.zip](https://github.com/user-attachments/files/27897284/paver.zip)

### Expected behavior

an object/block with rounded corners and chamfered top edges

### Actual behavior

top layer rises up and exposes hollow inside of shape/block

### Development version About Info (in Safe Mode)

```shell
OS: macOS 26.5
Architecture: arm64
Version: 1.2.0dev.20260512 (Git shallow)
Build date: 2026/05/12 17:54:42
Build type: Release
Branch: grafted,grafted
Hash: 758570d19c79cc4cf8b1e198b5568e7263e8e6d8
Python 3.11.14, Qt 6.8.3, Coin 4.0.8, Vtk 9.3.1, boost 1_86, Eigen3 3.4.0, PySide 6.8.3
shiboken 6.8.3, xerces-c 3.3.0, IfcOpenShell 0.8.2, OCC 7.8.1
Locale: English/Austria (en_AT)
Stylesheet/Theme/QtStyle: OpenLight.qss/OpenLight/
Navigation Style/Orbit Style/Rotation Mode: OpenSCAD/Trackball/Drag at cursor
Logical DPI/Physical DPI/Pixel Ratio: 72/124.5/2
Installed mods:
* 3D_Printing_Tools
* 3DfindIT for FreeCAD 1.2.0
* Assembly4.1 0.60.2-0.1
* Behave-Dark Colors 0.1.1
* Better Tool Library 0.9.9
* Bill of Materials Workbench 1.1.1
* Built-In Preference Packs beta 1.2.2
* Cables Workbench 0.3.5
* ColorPalette Theme 2.2.2
* Curved Shapes 1.0.14
* Curves workbench 0.6.71
* Defeaturing Workbench 1.3.1
* FEMbyGEN 2.5.5
* Fasteners Workbench 0.5.51
* Frame and beams workbench 0.1.1
* FrameForge 0.2.1
* FreeCAD-themes 2025.11.25
* FreeGrid Workbench 2.2.0
* Gridfinity Workbench 0.12.4
* Lattice2 Workbench 1.1.0
* Lithophane
* Maker Workbench 1.0.1
* OpenTheme 2025.5.20
* ProDarkThemePack 1.0.0
* Reporting
* SheetMetal Workbench 0.8.11
* Woodworking 1.0.1.39285.1.1.1.2025.08.06
* slic3r-tools
```

### Last known good version (optional)

```shell
fc 1.2dev 2026-04-22
```