I’m a longtime user of Syncloud, recently switched from 32 bit to 64 bit on the same hardware (RPI4 plus external 12Tb WD disk).
When on 32 bit I was having idle disk (no rotation) when syncing was completed by the 2 clients of the Nextcloud app. Rotation was properly restarting when needed by file changes, etc.
Now that I test the 64 bit version, with Nextcloud but no clients connected yet, I see that the disk never goes idle. But only when an internet ethernet connection is active: if I unplug the ethernet cable the disk goes idle after a few minutes.
Can you see in top what is happening?
Also you could install tcpdump to see if there are more requests than usual.
With and without the internet using attached screen.