FCP renders the same content over and over. Why?

I have a new mac studio, M1Max, 64gb RAM, 32 core graphics, running Monterey, and was hoping for better performance, but FCP is constantly rendering. Yes, I know you can turn background render off, but that's not the issue. My videos can be complex with multiple layers and the real time playback is laggy and poor resolution.  I need to review the rendered content in order to effectively edit.  I'll do a project, and it will fully render. If I come back the next day, it renders it all over again. Why can't it render an edited sequence once, and just use that rendered file from then on unless I re-edit the sequence? Over time my rendered files folder in that project will get huge, 600GB or more, and I just have to manually delete it because my hard drive is full. Then I get to render it yet again. What gives?

This is a known bug since many releases. Under certain conditions (i.e. most conditions, if not all) FCP does not retain render file information in cache upon restart, shutdown, etc. You can delete those render files within FCP. You mention deleting them manually - to me that means going into the library structure (show package contents) and manually selecting the render files and dragging them to the trash. That works also.

This has been driving me crazy for years - since El Capitan (10.11), and across 5 iMacs... I got tired of manually deleting the giant cache-files and finally turned-off background rendering and now manually render only small sections as needed.

Struggling_Editor wrote:

This has been driving me crazy for years - since El Capitan (10.11), and across 5 iMacs... I got tired of manually deleting the giant cache-files and finally turned-off background rendering and now manually render only small sections as needed.

I find it hard to believe that Apple just ignores such a glaring problem.  I guess they consider FCP complete and aren't doing any updates?  It really slows down my workflow.  This makes me think I should look at a different video editing suite.  

I've been using FCP(X) since version 10.3, heavily since 10.4.2, and haven't experienced the behavior you've described. This includes using FCP on a 2012 Mac Pro, various MBPs, and a 2019 Mac Pro.

I wonder if the issue could be related to something about your macOS setup. Perhaps, something having to do with drive-folder-file permissions or ownership, or Security & Privacy settings in System Preferences?

It's got to be very frustrating, no doubt...