I can't tell you why it's wrong, but I can help you fix it. Edit the feature and click on the blue arrow at the top of the feature's PropertyManager (it's pretty easy to overlook).
That will take you to the second page of the PM, where you'll get a preview of the Mirror, and you can select which orientation you want. Below is the default in the Mirror I used for the example.
When I clicked on the double caret at right in the box this is what I got.
If you've mirrored multiple components you may need to click on each one in the Orient Components box and fix each one.