Exporting a film using 5.1 sound and Prores 4444 format?

Hi, received some final files for a project. We are going to sync / export the final soundtrack with the final picture in Premiere Pro. Been sent a stereo mix and a 5.1 mix. The stereo track worked fine with Prores, exporting worked with this... but the 5.1 option doesn't appear. Is there a setting we have missed out somewhere? Any help greatly appreicated!

Thanks