project file thumbnails missing

It’s easiest if the cache is stored externally. Select the library in FCP. In library properties click Modify and the change the cache to the folder wherever the library is stored. It will have the same name as the library and be right next to it. Quit FCP. Move the cache to the trash and relaunch FCP. It will automatically rebuilt the cache bundle, restore new filmstrips and poster frames. Trashing the cache bundle is also an easy way to Trash Generated files for the library when you need to clean up.

Select the library in the browser, choose File->Delete Generated Library Files... Choose All Render files.

Manage render files in Final Cut Pro for Mac - Apple Support 

Unfortunately you can’t set the poster for the project. Try dumping your cache and restarting and see if the application builds posters that aren’t in the beginning of the title or the first frame of black.

As Tom said, I formerly thought the thumbnails for the project icon were taken from the thumbnail cache inside the library in /EventName/Render Files/Thumbnail Media. That is where the timeline thumbnails come from and I think for filmstrip clips in the browser.

However I did some brief tests that imply the project thumbnail is not taken from that cache.

I can add a clip to a new project, and the project thumbnail will remain black. If I click on a few things or switch several times between filmstrip view and detail view, it will fill in. Using the terminal command fs_usage to capture all I/O FCP is doing, it does not access the cache thumbnails when that happens. It does getattrlist calls on the original media files but does not read or write I/O. So it is apparently an in-memory state change.

When the project thumbnail fills in, these are the sequence of FCP calls. the 'switchDisplayViews' method is when it switches back from list view to filmstrip view. It checks to restore the previous filmstrip view selection, then calls the 'reloadData' method. That call may be what fills in the project thumbnail.

-[FFOrganizerFilmstripModule switchDisplayViews:]

-[FFOrganizerFilmstripViewController matchSelectionToPreviousModeSelection:]

-[FFOrganizerFilmstripView activeSelectionDidChange]

-[FFOrganizerFilmstripView reloadData]

It's possible upon initially adding the clip to the project, there is a code path which fails to populate the thumbnail. However I don't know why some users report it as widespread and consistent behavior.

Please post screenshots of what you’re seeing where.