my uneducated guess is that perhaps it's mirroring the points of the stroke without also mirroring the orientation of the brush to the stroke? anyway, hard to be sure but check out a screenshot here - https://drive.google.com/open?id=1ywjr6AE940QrunP5_TsO2RNq2w3c1Ve8 - that looks different, right? (the object being mirrored is that light square with a dark triangle on). As you can see it looks almost right but I don't think quite right?
my uneducated guess is that perhaps it's mirroring the points of the stroke without also mirroring the orientation of the brush to the stroke? anyway, hard to be sure but check out a screenshot here - https://drive.google.com/open?id=1ywjr6AE940QrunP5_TsO2RNq2w3c1Ve8 - that looks different, right? (the object being mirrored is that light square with a dark triangle on). As you can see it looks almost right but I don't think quite right?