Update:
Apparently Yuna is aware of the reversal of images because in the resources of Plus! there are some 'special' RTL alternatives for some images (eg: plusicon-16.png <-> plusicon-16-rtl.png), it's the same image as the original one but reversed (thus they will be shown correctly when used in RTL environment).
There is even a special RTL alternative version of the button logo talked about in this thread. But it is the same as the normal logo, it is not reversed (plus-icon-big.png <-> plus-icon-big-rtl.png)!!
So, I think this is a booboo on Yuna's side....