Here's a similar occurrence listed on the Apple site:
discussions.apple.com
One response: (More at the link)
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Hey there!
Yes that’s a bit much for system data, I don’t think I saw any extra partitions using much data either. Any other Users on your Mac?
While in the ~/Library, (That you are familiar with). Can you get the size of Containers? And if you also have a folder called Screen Recordings? There are a few others to check out.
Ive found if Apple Mail is used and Mail logging is enabled, it can continuously generate logs you don’t need.
To check:
Open Apple Mail app.
Click Window > Connection Doctor.
Click “Show Logs”
Click Command i to see the size.
If large delete everything in that folder, empty the Trash, and make sure to disable the option for Mail Logging in Connection Doctor, to prevent this again.
(These logs are deep within ~/Containers/com.apple.mail, so Connection Doctor is an easy way to find them).
I take it otherwise you’ve already also checked in the ~/Library:
Cache
Containers
Group Containers
Logs
Messages
Mail
Screen Recordings
These are my go to’s for Other/System storage, just don’t go delete anything as they may contain important data, however things like Cache and Logs, you can typically get rid of if they get out of line.
Hope that helps!"