Hi9 internal memory mystery memory usage

My Hi9 Pro is showing memory full, which it shouldn’t be.

If I look at the details page I see this:
Apps - 11.38 GB
Images - 0
Videos - 0
Audio - 0
Other - 0
Cached data - 6.44 MB

The total: 53.60 GB!

How is this possible. Where is the other 40 odd gigabytes and how can I get rid of it.

Last time I got to this stage, I performed a factory reset, which fixed the problem, but it has come back again and does not seem related to how many apps I download.

Any ideas, please?