Not sure if a bug as I could not replicate it.
Simple edit of 4 minute video which I needed to trim the start and finish and boost the volume.
After adding to the timeline I found the audio was out of sync with the video (This sometimes is the case with the recordings I get).
So split the audio and adjusted the relative positions.
The audio was also badly distorted - clipped / echoing.
I exported the audio to edit separately in audacity as it was so bad. To my surprise the audio was fine, just a bit quiet.
I did a test export of the video and the audio sounded fine, but was out of sync.
I re-edited the video keeping the audio with the video and this export was fine, even though the preview still sounded bad and out of sync.
I closed shotcut and reopened the mlt file and preview and export was now working fine.
So a strange one and I could not replicate it.