In my reading & experience, that space is often stale cache, e.g., iMessage attachments. I’ve not found a way to simply delete it. A device wipe & restore should free it up. I hate that advice, but it consistently works for me.
You could also try to leave that screen on for a few min, close that screen and reopen it again. They are most likely Apps, or Apps cache that wasn't calculated when you first use that screen.
There is also the iMessage bomb and iCloud Photo bug. But those were a long time ago and I haven't heard about it for years.
So yes best to do Factory Reset. And I know it is a pain. I have lots of people around me who just refuse to update apps and iOS anymore. ( But Apple is now actively courting them to do so )