FCP renders the same content repeatedly

This behavior is sometimes seen, but I don't know of well-documented parameters that cause it. In general if a range in the timeline is cached, it should stay that way until something in that range is edited. If it's a multi-layer stack, any changed clip or effect in that range will invalidate the cache over that range for the full stack.

E.g, if you have an adjustment layer over the timeline, anything that tweaks that adjustment layer will invalidate the cache for the entire timeline.

However if nothing changes, just relaunching FCP should not cause it to become uncached.

There are separate render caches for full-res media and proxy media, but simply switching between proxy and full-res modes should not invalidate cache regions.

I have seen this happen several times, but I personally haven't seen it happen over the entire timeline. Do you have any adjustment layers, titles, generators, or other items that span the entire timeline? If so, as a test, could you temporarily disable those ("V" key), render the timeline, and then relaunch FCP? Does that make any difference?

You might also turn off background rendering, delete the render cache, and then manually render the timeline using CMD+A to select all clips and CTRL+R to render. Then relaunch FCP and see if it makes any difference. In theory, that shouldn't make a difference, but sometimes background rendering seems associated with problems.

Make sure your FCP library and all media are on a locally-connected drive formatted as APFS or MacOS Extended Journaled. If some other disk format, it's conceivable that FCP might lose track of the render state, but that is speculation.

There are internal functions within FCP that track the render cache state. Here are some guesses of things that might affect this:

• Background processing: as stated above, turn off background render. Consider what clips have functions associated with background processing such as stabilization, optical flow, etc.
• One method FCP uses to determine render cache validity is the MD5 hash, which tells if the cache file changed since it last used it. You have no real control over that, and it should be independent of the filesystem. However, if something touched the cache file (maybe a backup app), that might change the MD5.
• Disk space: if inadequate disk space exists, the render cache might become invalidated due to the inability to re-render it. FCP never deletes cache files; it only creates new ones. If it can't do that, the timeline might revert to a non-rendered state.

I'd start by deleting all render files. A stuck render file could cause this looping error.

Also: if you haven't already, turn of "background render" in Final Cut Pro->Settings->Playback and leave it off always. Only render if FCP is unable to play a section of your timeline smoothly.

As a test, Snapshot the rendered project before you quit the application.

When you re-open the application, load the Snapshot to the timeline. Does that require rendering?

Are there any storage device changes while the application is quit - and/or are there any network storage devices involved in your set up?

MtD