Ok, checked and it is not a bug. Turns out that (like so many things in 3D), there is no actual standard for what a cubemap should look like. Some viewers want mirrored, some don't, so we had to pick one. We've gone for the one that is as common in viewers as the alternative, and it also matches our sister engine, V-Ray. While it may look odd when viewed in Photoshop (or the VFB), it will be fine when in a viewer that expects that (and ultimately, in the viewer is when it counts) - but since there's no standard, not all viewers will want that, and you'll need to mirror it if it so happens you are delivering for a viewer that doesn't want mirroring.
Cheers!