Hi @s.andraos
This isn’t a bug, but just how planes are defined in Rhino. I agree it can be a bit counterintuitive at first, particularly when it comes to mirroring, but once you know what behaviour to expect it does make sense.
Planes always maintain their handedness - Z is determined by X and Z, not something you can set separately.
Note that in your first example, if you look at the Z axis of your vertical plane, it is not getting mirrored in the World XY plane, but gets reversed:
