I observe that when mirroring faces the direction of the normal vector of original and mirror image is different. This can be seen when extruding. The user needs the same orientation of original and mirror image.
I have attached the construction steps in pictures.
Can the user correct this error himself?
i see that this could be modify in the salome behavior. but in any case, why mirror from a line? it is more rigorous to mirror with a plane (third option of symmetry), if you do that you will have the attended behavior, with normals in the correct direction. also another approach that could be better is to do the symmetry in the sketch itself, ie. have the two squares in the sketch itself and then directly extrude them, this also will solve your issue.