Rendered areas in my timeline disappear after a while

and have you tried "render and replace?"  You can always go in and change your edit if you need to...

I had initally been rendering to a single drive SSD, but that had exactly the same problems.   I guess I can dig out another Thunderbolt drive and try that.

My understanding is that software raids are less dependable than hardware raids, bwdik.  strongly suggest you try rendering to a non raid to see if that solves the problem.  If it does, then we can try and figure out what's going on...

Just this very moment, I spent 20 mins rendering the entire film - all green, fully rendered.   Save All, then Quit.   Without doing a single other thing, I immediately reopened the same project, and 1300 previews have disappeared and need rendering. 

RAID is a Thunderbay4 SoftRAID, formatted MacOS Journaled.   Permissions are set to be ignored on the RAID.   Both Previews folders have lots of render files, as expected, and there's 20TB free on the RAID.

I'm wondering if the issue is the external raid...    Can you check to see if the preview files are actually still on the raid?  They should exist in the folder you've set up in your project settings scratch disk...  Also, you might double check that the permissions for that folder are correct...  How is the raid formatted?    Maybe, as a troubleshooting step, try changing the scratch disk for video previews to the internal raid see if that solves the problem.  

All media and renders are on an external RAID.   All Previews are ProRes Proxy.

Sorry. Are video preview files on an external drive? A NAS? Any details on the disk you render to? If nothing is unusual, try changing the video preview codec to ProRes 422. Any change? 

Hopefully, I can get the product team to weigh in.

Thanks,
Kevin

what about trying render and replace as a workaround...  

Moved the entire Production to a Mac Studio, rendered everything, and a day later most of the renders have disappeared.   So it's not architecture based.