red thumbnail appears on fcp timeline

we're in the middle of an edit. nothing very complicated. but at certain zoom resolutions we're getting red thumbnails. it's not conisisten – if you zoom in further the red disappers. the footage plays back fine and exports fine, it's just very unsettling to see what looks to me like missing footage (even though it's not).

any brilliant ideas/solutions welcome.

I'm having odd behaviors with clips showing offline when I alter them in the Timeline.  There's some bugs in 10.5.1 need to be worked out.

maxhowarth wrote: we're in the middle of an edit. nothing very complicated. but at certain zoom resolutions we're getting red thumbnails. it's not consistent – if you zoom in further the red disappears...

The thumbnails are rendered to segments within the library: EventName/Render Files/Thumbnail Media, whereas the viewer data comes from either the source files or the render cache in EventName/Render Files/High Quality Media.

It should use the cached thumbnails, and if zooming in/out or resizing the event browser filmstrips or timeline, under certain conditions they are regenerated. They obviously should not appear as red.

Unfortunately there is no FCP UI method to delete the thumbnail cache. However, you can use the library inspector>storage locations>modify settings>cache to define a folder for cache, which will isolate render cache, thumbnails, waveforms, and optical flow files to a single cache bundle. That can be safely deleted from Finder without messing around inside the library. FCP will then regenerate those cached items as needed. 

I haven't seen this but as Ben said, it could be a bug. If the physical location for the library or cache not on a directly-attached disk formatted as either HFS+ or APFS, try that. Make sure you have plenty of extra disk space on all volumes.

As usual try things like resetting FCP preferences (item 11 on this list): support.apple.com/en-us/HT203477
If by some chance you haven't rebooted the machine recently do that. If the problem comes back, note how long from the last reboot. If simply re-launching FCP and not rebooting the OS temporarily fixes it, please report that.

that's really helpful, thanks so much. i'll try this out and get back to you.