Custom Poster Art, embedded in MP4, looks pale and washed-out viewed in PLEX

It seems, lately, the the custom poster art, that I embed with the metadata of my MP4 videos using Subler, looks pale and washed out, when viewed in Plex Media Server and in the Samsung and Roku Plex apps. The only thing I can do, for a nice image, is to use select one of the community posters for the artwork.
If I extract the poster artwork from the .mp4 file to a .png file using ffmpeg, or copy/paste from Subler, the colors look normal viewed in a browser or graphics editor.