A constraint is applied with its first-picked entities held and the second
moving; when the second can't move, a first one moves instead. That fallback
was skipped as soon as ANY first entity was locked, so a free construction
line between two fixed lines was never tried, and Symmetry was refused with
"neither entity can move".
Each first entity that can move is now tried on its own, after the second.
Nothing changes where the second can move.