Having the same issue. I just deleted the cache files and I no longer had the issue. Once I quit Premiere and relaunched the issue was back. I'm also experiencing another issue which could be related...Once the project is open it take about 1 minute before playback works. If I hit spacebar or the play button nothing happens. It seems like the media is still loading. After a minute Premiere will playback just fine. This is not normal. Also I just imported footage of the issue that I took on my phone and now its happening in the source monitor. I would say this issue is not solved.