How to reproduce:
1. Create a 3D project
2. Download the “M_Rock_01_World.fbx” asset
3. Drag & drop the downloaded asset to the “Assets” folder in the Project Window
4. Expand the uploaded asset and select the “M_Rock_01_World (Mesh)”
5. In the Inspector window, select “Change display mode” and set it to “UV Layout”
6. Change the channel to “Channel 1”
7. Observe the preview
8. Repeat steps 1-7 in 6000.0.62f1

Expected result: Generated Lightmap UVs on Channel 1 is the same in packing, island count/orientation, and margins in both steps 7 and 8
Actual result: Generated lightmap UVs on Channel 1 show noticeably different packing, island count/orientation, and margins between steps 7 and 8

Reproducible with: 6000.2.3f1, 6000.2.13f1, 6000.3.0f1 (820bc0c5f094), 6000.4.0a5, 6000.5.0a1
Not reproducible with: 6000.0.62f1, 6000.2.2f1

Reproduced on: Windows 11
Not reproduced on: No other environment tested