Hi,
To know the issue more clearly, would you please tell all the GPOs you set related to the screen saver?
If possible , please share a screenshot of the gpresult /h report.html
Did the registry for screensavertimeout was changed correctly?
Also,did you deploy the group policy :Force a specific default lock screen image under Computer Configuration > Administrative Templates > Control Panel > Personalization
What's the version for all the WIN10 workstations?
Based on my research, In Windows 10, version 1607, the Group Policy :Force a specific default lock screen image apply only to Windows 10 Enterprise and Windows 10 Education.
Group Policy settings that apply only to Windows 10 Enterprise and Education Editions
In this situation , you can consider Change Lock Screen and Desktop Background in Windows 10 Pro through a script
Best Regards,