Upgrade transform_conversions.c::set_prop_dist(...) to check all selected vertices when finding nearest selected vertex.
The current implementation of the set_prop_dist only looks for the nearest selected vertex to an unselected vertex from the same object as the unselected vertex. The new implementation looks across all objects for the nearest selected vertex.
I found this bug while looking in to why multi object edit proportional editing wasn't working as expected.
You will need diff D3992 to test this code. Also note that at the date of this diff, any objects that are being edited that don't have any vertices selected won't have a TransDataContainer - in other words if an object doesn't have at least one selected vertex it will not be affected by proportional editing.