Premiere doesn't multiplex an H.264 on export

I’ve tried several times, including quitting and relaunching Premiere, but in version Beta v27.0.0.38, when I export a H.264 using one of the built-in “Match Source - Adaptive Medium/High/Low Bitrate” settings, it does not multiplex the audio and video - I’m left with a mute .mp4 file and an audio-only .aac file.

The workaround is to export as a QuickTime ProRes and convert it to an mp4 in Media Encoder which thankfully still works ok.

Steps to reproduce:
Export any sequence as an H.264.

Result: Audio and video are separate files.

Expected: Resulting mp4 should have audio and video muxed.
 
System info
    Application: Premiere Pro (Beta) v27.0.0.38
    OS: Windows v10.0.26200, RAM: 127.91 GB GB, CPUs (logical): 32