Hi Remboy,
I understand the frustration you experienced. Sorry for the inconvenience.
The expected behavior is as following:
1.If we check the button in OneDrive settings -> Office -> use Office 2016 to sync Office files that I open, the changes will be synced in Office sync mechanism. So the changes will be saved to SharePoint first.
2.If we uncheck the button in OneDrive settings -> Office -> use Office 2016 to sync Office files that I open, the changes will be synced in OneDrive sync mechanism. So the changes will be saved to local OneDrive. If we have internet, then sync to SharePoint.
We know the second scenario is your requirement. But your issue is even if you uncheck that option, changes will be saved to cloud first (Online version). We have checked this in our side, but we can't repro.
For the current situation, you may try following steps to see if there is any improvement:
1.Right click the sync icon -> settings -> Account->Unlink this PC.
2.Log into the Windows system as Administrator > open command prompt > run taskkill /f /im OneDrive.exe
3.For 64-bit Windows, run %SystemRoot%\SysWOW64\OneDriveSetup.exe /uninstall to uninstall OneDrive (For 32-bit Windows, replace SysWOW64 tosystem32)
4.Access this path: C:\Users\yourname\AppData\Local\Microsoft\OneDrive and make sure all documents and folders are cleared.
5.Reinstall OneDrive. The latest version is 17.3.7294.0108
6.Untick the option "use Office 2016 to sync Office files that I open". Then test again.
If the issue persists after installing the new version of OneDrive sync client, we notice your SR has been escalated to the escalation team, you may monitor that for any new updates.
Regards,
Sky