Delete the current Mirror Feature from the Feature tree.
Create a new Mirror feature.
Select the face/plane you want to mirror about as before.
Expand the Mirror Bodies drop down in the Mirror Property Manager.
The Mirror Bodies option mirrors the complete geometrical shape of the solid body selected, rather than the specific features.
The bodies to mirror can be selected from the graphics area or from the Solid Bodies folder in the feature tree.
Under the Options drop down within the Mirror Property Manager, the ‘Merge Solids’ box may need to be unchecked if there is free space between the original body and the mirrored body.