Attention Fire Stick users. Are you constantly finding yourself running out of storage? I just noticed something less than a week ago. I had 503MB (.5 GB) showing in my screensaver cache. I deleted that cache and immediately found that my storage had increased by that much. Just checked it again today (less than a week) and saw the screensaver cache was back up to 401MB. Sure enough, I deleted it's cache and storage increased again. I just turned my screensaver off so I am hoping the cache will stay at 0.
Just a heads up if you constantly run low on storage. Delete your screensaver cache and see what happens. This is on my 4K FS but I would guess most models would be similar.
I had the exact same experience as you, and also did the same thing. With screensaver set to not ever come on, there shouldn't be any cache buildup.