I ran into this a few months ago, and that’s when I discovered that a feature that is common in every other image software I own, including my phone, is not available in PL standalone. And apparently it didn’t even become available until ViewPoint 4.
The Flip and Mirror tool is part of DxO ViewPoint 4. If you’ve activated DxO ViewPoint 4 on the same computer, the Flip and Mirror tool will be available in the menu lmage > Orientation and by right-clicking on a thumbnail in the image browser. This option is not available in older versions of DxO PhotoLab.
Honestly, this is nuts. I have PL8 and VP3.