Souce video is H.265 HEVC from ProCamera app running on iPhone 11 Pro Max. I have never experienced any problems with this source format in Premier Pro before, only a few problems in the past with Premier Pro taking too long to trim and transcode video to a lower H.265 bit rate for upload to YouTube and Facebook within their limitations.
Source videos range in length from 15 to 90 minutes. The longer the video, the more Premier Pro has been prone to problems. But this time I'm stumped. Trying Adobe's default 4K setting for HEVC video export, and trying to export from a 75-minute 4K 30P source video twice, I get the same buggy HEVC output result with the sound missing after several minutes of video.
On the PP2021 video editing timeline, all the sound plays back great, and the editing experience is very responsive on this new M1 MacBookPro. Prior HEVC exports worked fine on this M1 Mac except another longer and larger source video that I finally had to trim and transcode in Final Cut to get the result I needed.
With PP2021 unable to properly trim and transcode my video captured today, I'm again having to shift to Final Cut to trim and transcode. Nothing else needs to be done to the video, so I expected PP2021 to handle the job easily.
We're sorry about the poor experience. We can get this checked. Please let us know the exact version of Premiere Pro that you are using. Also, have you tried exporting in any other format & checked if it's exporting properly?