Sharing this solution for future devs that run into this problem:
EncoderManager.exportSequence() returns undefined and does not provide any error messages after the latest PPro Beta update.
The solution is that you need to also install the Beta version of Adobe Media Encoder. Non-beta AME is currently on 25.x and beta AME is 26.x. It seems like the version number of premiere and AME need to match to get audio exporting to work. Before this wasn’t an issue because Premiere Pro Beta was still 25.x which matched non-beta AME 25.x.
This may also happen with video exporting but I haven’t tried it.