I too am experiencing this since upgrading to Big Sur and then to the latest version of Premiere. Sometimes, I'm not getting any of the audio tracks from the nested clip. They might pop back in if i allow them to continue to play past an edit in the source clip, but not always, and weirdly sometimes only partially at a faint volume.
Other nested sequences only play audio 1 when I've allowed the main sequence to play from nested clip to nested clip. But if I pause and then continue, I get all of the track playing again.
And muting/unmuting and/or soloing/unsoloing each of the audio tracks in the source clip sometimes works, but not always.
I'm wondering if it's a problem reading different sample rates? If the main sequence and nested sequences don't match, or the audio laid into the nested sequence is different than the sequence settings. Can you not mix sample rates?

Same problem... Working with latest Premiere version on multi-cam sequence that contains multi track audio...
If I start playing before the clip, it will only play a single random track. If I start playback at first frame of the clip, all tracks are played. Behavior changes if I add another clip or move them around.
Rendering audio preview doesn't fix the problem. Exporting is fine... Go figure!

Also having this problem, so I've been copying the sync'd tracks to another timeline, then nesting my video so I can keep all of the audio tracks in there too.

Same here. When nesting a 2nd sequence into the main, there is no audio heard from the nested sequence - only the main track. I even put the 2nd sequence audio on a different track than the main and still no audio. I guess I have to bring all audio into the main and re-sync which is a huge pain.

We also seem to experience this issue. Seems to have been around a while: http://www.videoforums.co.uk/threads/25063-Premiere-Nested-Sequence-amp-Audio

I am editing a feature and just started experiencing the same issue,. Major workflow issue.