Asymmetric skins are not reflected

I'm Japanese and not good at English, so please forgive me for using automatic translation.

I tried to make my own "Alex" type, asymmetrical skins(zm - コピー.png).
The first image is the skin that I made myself. It was created according to the template, and the overlay was made using only "Jackets".
And when I tried to reflect this skin, the design of both feet of the skin was unintentionally unified(Screenshot).
I looked into the solution, but what came out was only the voice of being troubled by similar symptoms, and I did not reach the cause. On the other hand, the second image used for the template was output in a normal state with different designs on the left and right(3px.reference.png).

Even if this isn't a bug, I think it's a trouble that the specification isn't fully explained. Please explain.