Do you just have the one client device? Do you leave Plex open? On which screen in the Plex app, if so?
Yeah. I think there’s a bug in Plex’s cache filename generation logic.
It generates different cache filenames very often (or always?) for the same source and content.
In addition to the wasted storage space, this means wasted time calculating and saving the cache images. It also means the cache doesn’t work well (at all?) as a cache, when the previously-generated files aren’t used.
Plex will eventually delete old files.