Hi Jolfs,
Thank you for reaching Microsoft Community. I'm sorry to hear about the dGPU issue that you are having on our Surface Book 3. We appreciate the efforts and time in doing the troubleshooting prior to contacting.
It really is weird that the NVIDIA graphics shows it is working fine in Device Manager but not being detected in Task Manager or other applications. May I what is the Driver version of the NVIDIA graphics that is currently installed on your Surface? See this under the Driver tab of the NVIDIA graphics properties in Device Manager.
Kindly try first running the System File Checker Tool on the device then see if the issue persist. See: Using System File Checker in Windows (microsoft.com).
If in case the issue persist, proceed in downloading and installing the MSI driver package for the Surface Book 3 by following the steps below:
Check what is the current OS Build of your Windows version by going to Settings > System > About.
Go to Download driver and firmware for Surface, choose Surface Book
Then click the link next to Surface Book 3 then click Download on the next page
Choose the .msi file that matches your current OS Build and click Next
Open Device Manager, expand Display adapters
Uninstall the INVIDIA graphics adapter
Note: If prompted with a checkbox stating "Delete the driver software for this device", check the box then hit Uninstall.
Run and install the MSI downloaded earlier then Restart
Hope this helps.
Kind regards,
Marrion