This is an issue I have come across. I am trying to use a render texture to capture a section of a sprite using a sprite mask to set the size and shape of the render texture output. As the render texture output has no sprite mask info the output is nothing. The render texture works fine with sprites not interacting with sprite masks. As stated above this is not a bug but an unsupported feature. I would really like to see this feature supported as it would allow for the use of sprite masks to select from large sprites out of the way of the main camera in the scene, without having overlapping sprite masks messing each other up.