Sometimes AutoCAD does stupid, illogical, unexplainable things.
Don't try to overcome it, just learn to work around it.
Since you have one "wing" unioned, my suggestion is to slice the center piece in half, vertically, delete all objects on the side lacking union, then mirror the 'good' half, and union those halves. That should hopefully work.
AutoCAD does stupid unexplainable things like that, often.