Basically this Black Covers or previously White Covers never get solve since it appear.
It appear from time to time.
I am using Plex Plugin for Infuse in Apple TV 4K and custom cover most the time.
I have 2978 movies and 2933 TV episodes for the moment. Metadata size is 3GB.
Short solution - change cover at Plex and refresh at Infuse, later change back prefer cover and refresh again at Infuse.
This is kind of annoying already.
Before it happen, it work perfectly fine, now from time to time this happen, when this happen, after sometimes it also happen cover load very slow, when that happen, re-generate all metadata is required. Also when all this happen, crash will happen from time to time.