Hello there,
Welcome to Microsoft Community.
The screen split feature of the Surface Book series will only disconnect the base GPU when you press and hold the detach button and see "Ready to detach" on the screen.
If your SUrface Book3 disconnects only when touched or vibrates, there is a problem with the base and screen connection of the device.
If your Surface Book3 is out of warranty, there is a fee to create a repair.
For instructions on this and how to create a repair, please refer to the following two links:
Out-of-warranty product repair costs:
How much does out-of-warranty service cost for your Surface device or accessory? - Microsoft Support
Surface Creation Repair Service:
How to get service or repair for Surface - Microsoft Support
Thank you for your patience and understanding!
Reagrds,
Manson | Microsoft Community Support Specialist