I'm using 10.4.10 on a 2017 iMac Pro with 64GB RAM but only the Vega 56. I've been having this issue since at least the beginning of 10.4- though I should add that I do have a number of plugins being used on many clips. Most prominently Color Finale. However, I find that every time I open a project it needs to be re-rendered. When I Render All it only renders a portion and the only way to render the whole thing is by turning on Auto Render. Any thoughts?
If you manually Render All it should normally render the entire project. If you touch the keyboard or mouse during that process it may halt the render. Try it without touching the controls.
There is a periodic issue where render of clips with optical flow may get stuck and refuse to render. Unfortunately there is no built-in FCPX menu for deleting optical flow files. You can do it with Final Cut Library Manager, or by deleting those files within the library (usually not a good idea). However if you click on the Library Inspector and use Storage Locations>Modify Settings>Cache and assign a folder, then you can safely delete the cache object which also contains optical flow files.
In general rendered cache files should persist after closing and opening a project or re-launching FCPX. However there are several complex scenarios where render files are marked invalid and render dots appear despite no edits. Another issue where render files do exist but FCPX doesn't use those. See this post for details:
I can assure you that Color Finale specifically is broken in the new FCPX 10.4.9/10 update. You have to pay to upgrade to the new compatible version. I'm not happy about it. But if you're using Color Finale, yeah, you got problems.
I am using the latest Color Finale 2.0 Pro version. It's been working fine, but the rendering has been an issue and now since version 10.4.10 I also can't Share to Vimeo. I can only output the file and then upload.