ES File Explorer - Space not freed after deletion from Recycle Bin

Every few months, my internal storage begins to fill up. I store everything I can to SD Card, but Android has a limited ability to do so. I'm running Oreo, unrooted. The biggest dir by far is /storage/emulated/0/DCIM/.thumbnails/, which can stand around 3.1GB for some reason. I used ES File Explorer to delete it, then went to the Recycle Bin in the app and emptied it. The 3GB was not freed-up. I've restarted my phone several times, looked around for residual files, could not find LOST.dir. I'm honestly at a loss. Where did it go? How can I get the OS to free up this space? What better way can I use to get rid of this stuff?