That seems like bad normals. I don't know how Blender render things in the viewports but some softwares ignore normals and render both sides of every polygon, that's would be why it looks right inside Blender but wrong in unity. The only way to make sure is to analyze the model inside Blender to see if the mirrored part got the normals flipped somehow. I don't know Blender to walk you through that though.
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