I don't really understand this behavior :
All is done in object mode. Here are 6 identical shapes, from top to bottom rotated around the Y axis, respectively 0, -45, -90, -135, -180 and -225°.
Now I scale them -1 with 3D cursor as pivot and along the X axis (shapes on the right are 'witness') :
Now, the scaled shapes, from top to bottom (all kept their original rotation around Y) :
- 1 : scale X = -1, effectively mirrored (like from a mirror)
- 2 : scale X = -1, but not mirrored ?
- 3 : scale Z = -1, and appears mirrored
- 4 : scales X and Z = -1, not mirrored ?
- 5 : scale X = -1, mirrored
- 6 : scale X = -1, not mirrored ?
Why all that ?
Note : I don't want to obtain any particular result here (I know how to correct things if needed), but I just want to understand why it works like that.