Media encoder won't render prores with alpha

I'm trying to batch convert a series of clips encoded in png with alpha .mov to .mxf with alpha.

Whatever I try the alpha isn't there.

I'm selecting Apple Prores 4444 with alpha in media encoder and double checking that the settings inside the preset are ok but whatever I do the Alpha is always lost. As I say, the original clips HAVE an alpha channel.

Again what was a quick job is not so 😞