I’m running into this issue intermittently as well. Sometimes the previews reload fine but usually only portions do… then I have to re-render which on a 15 minute piece featuring lots of Magic Bullet can be really time consuming.
I suspect the issue is related to my Editshare storage system. I created a share just for previews and render cache that shows up to the system as drive R: Now obviously if I were to change that drive letter (or not have the share mounted) I’d expect to lose the previews but I haven’t done so in weeks.