The global mirror in object mode inverts the scale of the object along the axis (assuming the object hasn't been rotated). This can be seen in the sidebar N in the Item tab. Applying the -1.0 scale causes the normals to face in the other direction along that axis. While this may not be the intention, this is the expected outcome of applying the negative scale factor to the mesh.

The normals can be quickly corrected by recalculating them outside through ⇧+N or Mesh > Normals > Recalculate Outside .