It doesn't do it all the time, just sometimes, within the same session. When this happens, a yellow line appears above the timeline, above the clips that show as blank on the preview.
Not sure if it matters, but I can't get it to render preview either. It gets stuck on 0, 4 or 76%, always in random order.
This is the free version to try it out. I have uninstalled and re-downloaded once already.
TIA!