That has also been noticed by supporters, if the "Other" gets too big then backup is not possible.
The extreme measure of manually importing photos and other media, documents etc into your Mac/PC, and using alternative cloud backups for WhatsApp etc may be needed.
Once your data is safe somewhere you need to do Settings, General, Reset, Erase all content and settings. And start again, Restoring backups may restart the bloat.
Senior supporters are still trying to understand why Other in some cases is not self-managing,
People hate it when I say "I don't have this problem", my Other is steady at under 4GB, but then I keep away from News apps, games and social media altogether.