FEM: Polygon selection in Erase Elements causes serious graphical artifact

### Problem description

Unfortunately, #31579 is not fully fixed. I still have it when using polygon selection of the Erase Elements tool. Perhaps other tools using this kind of selection also have it.

### Workbench affected?

FEM

### Steps to reproduce

1. Open the file from here: https://forum.freecad.org/viewtopic.php?t=106824&start=10#p901743
2. Double-click on the Analysis container.
3. Select Mesh in the tree, enable Mesh --> Erase Elements.
4. Click the Poly button.

It happens in Safe Mode too.

### Expected behavior

No graphical artifact

### Actual behavior

Graphical artifact

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

```shell
OS: Windows 11 build 26200
Architecture: x86_64
Version: 26.3.0dev.20260812 (Git shallow)
Build date: 2026/08/12 05:41:29
Build type: Release
Branch: (HEAD detached at dc1ef6e88)
Hash: dc1ef6e880d6f5cf5d5a6eb3749373141eb0deb8
Python 3.11.14, Qt 6.8.3, Coin 4.0.10 (bundled), Pivy 0.6.11 (bundled), Vtk 9.3.1, boost 1_86, Eigen3 3.4.0, PySide 6.8.3
shiboken 6.8.3, xerces-c 3.3.0, Clipper2 , IfcOpenShell 0.8.2, OCC 7.8.1
Locale: Polish/Poland (pl_PL) [ OS: Polish/Poland (pl_PL) ]
Stylesheet/Theme/QtStyle: FreeCAD.qss/FreeCAD Light/
QPA/File dialog/Color dialog: windows/via Win32/via Qt
Navigation Style/Orbit Style/Rotation Mode: Blender/Rounded Arcball/Window center
Logical DPI/Physical DPI/Pixel Ratio: 96/81.5973/1
OpenGL version/vendor/renderer: 4.6.0 NVIDIA 610.88/NVIDIA Corporation/NVIDIA GeForce RTX 5080/PCIe/SSE2
```

### Last known good version (optional)

```shell

```

Reproduce the artifact in the FEM workbench by opening the linked file, selecting Mesh, enabling Mesh > Erase Elements, and clicking Poly. Inspect linked pull request #31961 and compare its behavior with the reported Windows Safe Mode reproduction. Done means polygon selection no longer leaves the reported graphical artifact.

Written by the indexing model from the issue text.