I don't know about the preview issues, but I have also had some bizarre issues during encoding. Specifically h264 using the mp4 container will crash encoding al

I don't know about the preview issues, but I have also had some bizarre issues during encoding. Specifically h264 using the mp4 container will crash encoding almost 100% of the time. This started a couple of major versions ago, and continues to today. When encoding h264 in a mov container, no crashes, any other codec, no crashes. I would say simply try a different container and then use ffmpeg to copy / transcode into a format you want to deliver.