This issue does not occur when ProGrids is not installed, or when Toggle Snapping in ProGrids is off.
With Toggle Snapping on, this will still sometimes work as expected, but the issue seems to happen more often than not.

1. Create a new project.
2. Install ProBuilder through the package manager. (Window -> Package Manager)
3. Install ProGrids through the package manager. (Window -> Package Manager)
4. Ensure 'Toggle Snapping' is set to 'On' in the ProGrids Window. (Tools -> ProGrids -> ProGrids Window)
4. Open the ProBuilder window. (Tools -> ProBuilder -> ProBuilder Window)
5. Click 'Create a new Polyshape' and create a Polyshape with various gradients of sloping faces.
6. Click 'Create a new Polyshape' and try to create Polyshapes from the faces of the first Polyshape.
7. Repeat on various slopes of the first Polyshape until you have a new shape that floats.

Expected Outcome: Every new Polyshape should be built from the face's surface normal.
Actual Outcome: A new Polyshape will sometimes float slightly above the face of the Polyshape you are trying to build from.

Does not reproduce when ProGrids is not installed. Reproducible from ProGrids preview - 3.0.1 and above with any ProBuilder version 4.0.2 and above.
Not a regression if this is a ProGrids specific bug. ProGrids does not seem to interact with ProBuilder 3.10.1 or below.

Notes:
- Tested on primarily on Windows, could reproduce on Mac.
- Can't pin down an exact condition where an object will float instead of attach to the face. Seems to vary even with similar gradients. Will update case if I have any new information.
- Despite this exact condition for floating being uncertain, once you find a face where it floats, every polyshape you try to build out of that face will float.
- All ProBuilder versions appear to have this work perfectly when there is no ProGrids installed.
- Similarly this issue does not occur if ProGrids snapping is turned off.
- Video file and image of this issue attached.