Picking distance is too high making it difficult to select a different object closed to the one that is currently being edited.

1. Create two PB cube setting the position 1.5 units apart in space
2. Enter vertex selection mode
3. Select one cube to make the vertices display in the scene view
4. Hover the mouse on the second cube and click on it as if you wanted to edit the second one instead

Result
The first cube will remain selected and clicking on the second cube will most likely select a vertex on the first cube
Depending on the angle of view, we won't be able to select the second cube in most cases.

See attached gif.