Rendered areas in my timeline disappear after a while

Have you tried lowering the program monitor resolution to 1/2 or 1/4?  I'm not seeing any issue playing back 2 and 3 camera multicam material in proxy mode on a decent 3 or 4 year old intel imac...    no captions though.   

They range from 1080 to 720, all ProRes Proxy.

What pixel dimensions?  If you use the low or medium res prores proxy preset, it won't play without rendering?   

All dailies are proxied to ProRes Proxy.

it's never easy to diagnose problems from a distance...  and gotta say never occured to me to ask if you were dealing with a "production" and captions...  Both of which may be contributing to the issue..   Wondering if the workaround is to create lower resolution proxies in an all iframe format like prores so you can play and edit without rendering...   Personally I haven't worked with productions, but have been doing some research as I'm wrangling a large documentary project with extensive multicamera 4k interviews (thankfully all in English). .    I know workarounds are not ideal, but sometimes it allows you to get the job done.  In my experience, Premiere does a great job with medium and low resolution prores proxies... 

I agree, I see this as a new problem I hadn't ever seen before, SONER.   I keep testing things and it just seems completely hopeless, and I cannot find a clear association with a cause.   BUT, I am starting to feel that at least some of it is to do with the caption track.   I'm working on a feature doc that is entirely subtitled.   Just now, I rendered the entire film (90 min feature).   I fixed a typo I noticed in one subtitle, and of course it turned from green to yellow.   I hit option-R to render again, and 721 video previews needed to be rendered again throughout the entire film, not one, as would be expected.

That said, I have seen six projects in my Production lose ALL their renders since two days ago, when I rendered every single sequence fully.   And many of those sequences had no caption tracks.   So it's not just the caption track.

All my project files are located on the internal ssd of the Macbook. I have been advancing with the same working principles for years and this problem has existed for 2 months.

Not sure it's that much more time-consuming...  bwdik...  and probably a useful troubleshooting step...  

I haven't tried that because the timeline is too much 'in progress' and I'll just constantly be unpicking them. 

If you have tried rendering to a local SSD, I don't think the RAID is the issue...