Have I just found a big APFS bug?

[Wando64]

OK, I must admit defeat and recognise I have a problem I cannot solve on my own.

This problem is on a 1TB external SSD (not that it matters, but it is a Samsung T5)
I am running Sequoia 15.4

Disk Utility, or File Info from the Finder, both show the disk as being full (only 10GB available), however:

- Finder shows only 600GB of data (including any and all hidden files and folders).
- There are NO local snapshots. I've deleted them using Disk Utility, and restarted the Mac.
- The Recycle Bin is empty.
- There are no additional or hidden Volumes nor VMs on this disk.
- The data on the disk consists of Photos and music files, including the Mac Photos library.
- I've unmounted the disk and run First Aid successfully.
- There is no amount of purgeable data of significance (3.65GB)

What is going on?
Thanks

 

[Wando64]

Update: I have moved all of the visible data onto another disk and the SSD now still shows as having 434GB used space.
I cannot see anything at all in it with the Finder, including hidden files.
Still no snapshots and no purgeable data.

I am stumped.

EDIT: Daisydisk marks the space as Hidden, but it is unable to establish what's in it, if anything.
I am starting to think this is due to some kind of corruption, even though First Aid is not picking up anything wrong.

 

[svenmany]

Daisy Disk has the feature to run the scan as an administrator, assuming you don't have the App Store version. They suggest doing that as an option to reveal what's in the hidden space. Are you able to do that?

 

[Wando64]

Daisy Disk has the feature to run the scan as an administrator, assuming you don't have the App Store version. They suggest doing that as an option to reveal what's in the hidden space. Are you able to do that?

Yes I have and it still came out with nothing beyond it being hidden space.
The picture above was taken after scanning as Admin, with the paid version of the software.

I’ve now given up, erased the SSD and put the data back.