How to reproduce:
1. Open the attached "IN-125342_PBUVIssue.zip" project
2. Open the "SampleScene"
3. Make sure that "Auto Lightmap UVs" is enabled in Preferences -> ProBuilder
4. Select the "Cube" GameObject in the Hierarchy and select ProBuilder tool context
5. Open ProBuilder UV Editor window and select any face on the cube
6. In the UV Editor, click any of the following actions: "Group Selected Faces/Break Selected Groups/Select Texture Group"
7. Observe the Inspector window

Expected result: Lightmap UVs are built automatically and there is no warning in the Inspector
Actual result: "Lightmap UVs are missing, please generate Lightmap UVs" warning appears under the "ProBuilder MeshFilter" component, informing that already generated UVs were deleted

Reproducible with: 4.0.2 ( 2020.2.8f1, 2020.3.49f1) 6.0.8 (6000.0.62f1, 6000.2.13f1, 6000.3.0f1, 6000.4.0a5, 6000.5.0a2)

Reproducible on: Windows 11
Not reproducible on: No other environments tested

Note: The warning also temporarily appears when selecting faces in the UV Editor and starting to drag them, but disappears once dragging is stopped