Latest build does not multiplex on export

Steps to reproduce:
    1. Put a video with audio on a timeline

2. Export (or send to Media Encoder)
    
   Result: … 2files appear: MP4 with video, AAC with audio. They are not muliplexed
   Expected: ...A multiplexed file, just like what I got when I exported from the shipping version.
    
   System info
       Application: Premiere Pro (Beta) v27.0.0.36
       OS: Windows v10.0.26200, RAM: 63.79 GB GB, CPUs (logical): 16