How to reproduce:
1. Open the “LightmapUVBug.zip“ project
2. Open the “SampleScene“
3. Select "Mesh.065" and focus on it in the Scene view
4. In the Inspector, right-click the "Lightmap UV Gen" Component, and select "Generate Lightmap UVs"
5. Observe the Scene view and the mesh in it

Expected result: Some different submesh vertices are connected
Actual result: Different submesh vertices are not connected

Reproducible in: 2022.1.0a1, 2022.3.63f1, 6000.0.51f1, 6000.1.8f1, 6000.2.0b6

Reproduced on: Windows 11 Pro (24H2)
Not reproduced on: No other environment tested

Notes:
- Though this has not been tested locally, the user has mentioned that the problem can be worked around by removing the offset of the baseVertex before calling the method
- The mesh can be restored by right-clicking the "Lightmap UV Gen" Component and selecting "Restore Original Mesh."