Filleting on pockets on domes with flat tops is broken

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Problem description

I just started and dunno the technical words so here's a pic:
Possibly related to #17845?

To reproduce:
1. Make a dome with a flat top, I tested revolve and filleted cylinder. Doesn't happen on fully round domes.
2. Make a pocket that touches the flat top

3. Fillet or chamfer the pocket

### Full version info

```shell
OS: Arch Linux (xcb)
Architecture: x86_64
Version: 1.0.0.39109 (Git)
Build type: Release
Branch: makepkg
Hash: 2fcc5317fe3aee96ca73475986a577719fc78e20
Python 3.13.1, Qt 6.8.2, Coin 4.0.3, Vtk 9.4.1, OCC 7.8.1
Locale: English/United States (en_US)
Stylesheet/Theme/QtStyle: FreeCAD Dark.qss/FreeCAD Dark/
```

### Subproject(s) affected?

PartDesign

### Anything else?

here's 3 examples

[filletbug0.zip](https://github.com/user-attachments/files/18642778/filletbug0.zip)

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct