It’s not extensive but I occassionally notice that files I’ve added, later, disappear after later Library updates.
The file was properly added, when I first added it, but suddenly doesn’t show up anymore and won’t, at all.
In this particular case, I noticed the file for Happiest Season (2020) won’t get detected. I took some logs where I restarted the device, went to Movies, hit Update Library, and the file does not get detected: https://paste.osmc.tv/iquyivanok.
It did, but it saw it was already in your library and the date when it had been modified hadn’t changed…
VideoInfoScanner: Skipping dir '/media/Seagate Basic/Movies/Happiest Season (2020)/' due to no change (fasthash)
I also noticed that you added that movies folder as its own source location and that possibly could cause issues with anything in that folder being picked up depending on the scraper settings (which is not something visible via logging). After removing that source you might need to navigate to videos>files>movies> then context menu over “Happiest Season (2020)” and change/set content to fix that file path. If you see this behavior again first check to make sure it isn’t getting hidden by a view mode (like watched/unwatched only views) and barring that restart Kodi and see if it shows up then.