How to reproduce it:
1. Open the user's attached project "ProBuilderRotatedParentSelectionIssue.zip"
2. Load the "SampleScene" Scene
3. Drag a "Cube" GameObject into the "RotatedParent" GameObject in the Hierarchy window
4. Select the "Cube" GameObject in the Hierarchy window
5. Select the Vertex mode in the ProBuilder's Edit Mode toolbar in the Scene view
6. Try to drag-select the top-half "Cube" GameObject's vertices
7. Observe the selected (yellow) vertices

Expected result: Top-half "Cube" GameObject's vertices are selected
Actual result: Not all or not top-half "Cube" GameObject's vertices are selected

Reproducible with: 4.5.0 (2018.4.34f1), 5.0.3 (2019.4.25f1, 2020.3.6f1, 2021.1.5f1, 2021.2.0a15)

Notes:
- Not constantly reproducible. "Cube" GameObject frequently loses selection state
- Reproducible with Vertex/Edge/Face ProBuilder's Edit Modes