Is there any fix for MoUsoCoreWorker.exe preventing Windows 10 entering sleep?

According to the information supplied by the poster, it's most likely that Windows was waking up the computer at night in order to install new updates, as are the default settings of Windows Update.

Once all updates have been installed, the computer goes back to sleeping happily and doesn't wake up any more. Until the next time.

Finer control over Windows Update is found in Settings > Update & Security > Windows Update and in Power Options > Change plan settings > Change advanced power settings, under Sleep, Allow wake timers.

Just run this command in an elevated command prompt:

powercfg /requestsoverride process "\Device\HarddiskVolume3\Windows\System32\MoUsoCoreWorker.exe"

Note that the part in quotes needs to match the path from powercfg -requests