Nxt 5.5 alpha channel texture not working

Hi everybody, my evaluation version of nxt5.5 does not render material alpha-channel transparency.
Transparency appears correctly in the 3d model, and Rhino render engine renders it correctly too so I can’t get why Flamingo can’t render transparency.
Do I have to activate some option in render pannel?
Is there any known bug on flamingo?

I’m using Rhino6 evaluation
Thank you