Files not uploading on MacBook Pro via iCloud Drive (but will sync files created on iCloud.com)

I have had an issue for a while where any file I save to iCloud on my MacBook Pro (2019) that file will just sit there with an upload arrow and file transfer rate that doesn't change.

If I create a file (say a keynote) on my iPhone/iPad or on iCloud.com, the file will sync correctly and show up on all devices including the MacBook Pro.

I had ZERO luck with level 1 support at Apple. They had a look at my machine remotely and could see the issue first hand. Tried some server side reset to no avail. In desperation I had upgraded from Catalina to a beta of Big Sur at the time in attempt to see if the issue would resolve. But no luck.

I sort of ignored the issue as I primarily use Dropbox now (due to its reliability). However I always wanted a resolution and have started on trying to fix the problem again.

• Have done a clean install/hard drive format and installed Monterey.
• Have deleted all files & folders in iCloud Drive and let the system rebuild the file structure (incase it was a stuck file/folder).
• Turned off iCloud Drive and turned back on.
• Created a new user account and logged in with the same Apple ID.
• If I create a new Apple ID and try that it works fine, so I know its account related.
• Have tried some terminal commands (killbird etc)

At this stage I'm guessing it could be a gremlin with iCloud that Apple need to fix for me, however if anyone else has any ideas?