Steps to reproduce:

1. Create a new project
2. Go to Window → Package Manager and install ProBuilder package
3. Create 2 GameObjects using "Create PolyShape" tool in the Scene view
4. Select both GameObjects
5. Right click in the Scene view → ProBuilderMesh → Merge Objects
6. Edit the GameObject using "Edit PolyShape" tool in the Scene view
7. Observe the Scene view

Expected result: Both PolyShape GameObjects should remain visible and merge correctly when using the "Edit PolyShape" tool after merging
Actual result: One of the 2 merged polyshapes disappears

Reproducible in: 5.0.3 (2021.3.0f1), 5.2.3 (2022.3.57f1), 6.0.4 (6000.0.36f1, 6000.1.0b4, 6000.2.0a1)

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