Having two odd and similar problems that only happen sometime.
I click on a shape or other object to select it.
Instead of the object under the mouse being selected, another object that is an inch or two away (at 100% view) is selected. There is no overlap, no common grouping, these are two completely distinct objects.
A similar things happens when selecting text fields in a group of shapes. I double clock on a shape to select the text field, and instead the text field in a different shape in the group is activated. Sometimes if I repeat this it selects another different shape again. There is not a clear pattern. Again, these shapes are not overlapping (they are grouped).
Restarting Omnigraffle fixes this behaviour temporarily.
Environment is OmniGraffle Pro 7.12.1, MacOS 10.15.2, MacBook Pro 15" Mid-2015 2.8 GHz i7 Dual Graphics and using an external monitor.