Why is cached file not deleting automatically on my iPhone?

You are experiencing a bug that occurred that whenever you delete media files it doesn't do so properly and adds the now corrupted files to Other. Try connecting your iPhone to your computer and update to 14.7.1

What cached file? And if you deleted your photos did you then go into the Photos app and delete them from the Recently Deleted folder?

Other is not necessarily a cached file, both the system and some apps have data in Other. How large is Other on your iPhone and what version iOS are you running?

Yes.

By doing it via your computer rather than doing an Over-the-Air (OTA) update.