So you render preview files in Premiere and the next time you launch the program you're met with red bars in the sequence - indicating the need to re-render (even though those render files are still sitting on the hard drive right where Premiere last left them). While Adobe is purportedly aware of the issue, it's still pretty annoying.
I found a potential workaround which, so far, seems to work - at least for me.
Premiere seems to lose track of rendered preview files on the sequence that was displayed at the time the project was saved before exiting. When Premiere is re-launched, by default, it opens to the last displayed/saved sequence.
So here's what to do, whether your project consists of a single sequence or several.
1) Create an empty sequence. Leave it empty. Rename it if you like ("Select me before saving and exiting") - or not.
2) Work in your other sequence(s). Render your previews as needed.
3) Before exiting Premiere (this is the important part) switch to the empty sequence and save the project. Then exit.
4) The next time you start Premiere: a) Select the project, b) Wait till all media is loaded (Premiere reports this at bottom). It will open to the empty sequence. When you switch to the work sequences, you should have green bars where the previews were previously rendered.
Try it and let me know if this works for you.
Preston