White edges on the channels of a multimatte render element

[gagui]

I'm working in LWF, so I think the problem comes from there.
When I save my multimatte element from vfb and open it in photoshop I get, in each channel, a white edge around the shape of the other channels.

I think I'm doing something wrong with the gamma when saving the image, but I don't know what.

What the correct workflow to save multimatte elements in lwf?