We have same problem on each of W10 1909 or 2004 computer (about 500).
GPO is applied successfully, because when i delete all registry setting in HKLM\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate\AU , gpupdate adds the values back correctly.
It's interesting that "Auto Reboot will not happen with logged on users" in Windows update advanced settings is displayed even if it is not set (neither GPO nor registry).
And GPO/registry "No auto-restart with logged on users for scheduled automatic updates installations" / "NoAutoRebootWithLoggedOnUsers" have no effect on this function.
No other GPO or local policy has windows update set, so there is no conflict.
Edit: our screens
GPO CAU https://pasteboard.co/JyR1y15.png
Computer registry wu https://pasteboard.co/JyR1TdT.png
Computer registry wu-au https://pasteboard.co/JyR2kPK.png
Computer policies view https://pasteboard.co/JyR2GkC.png
Computer Windows update state https://pasteboard.co/JyR3mMb.png
User is always logged in.
I logged off the user yesterday afternoon, the computer was turned on all night.
The user logged in this morning and WU immediately began to install pending updates.
Then WU planned restart at noon.
So Windows Update ignores the settings in registry/GPO.