I tried uploading a pic, but it didn't work.
When using timeshift, it will record over the file in use. Every time a new timeshift session starts, a new file is created. This is how the cache can completely fill all available space on the external drive. When that happens, timeshift no longer functions due to a lack of space.
I'm deleting the files now as part of weekly maintenance. Maybe this will be addressed in a future update, or not. Just wondering if there was another way to empty the cache.