Hi, Bolkiman!
Thanks for reaching out to Microsoft Community and posting your concerns regarding your Surface Book 2.
Please see a table of the supported resolution and refresh rates for Surface at the https://support.microsoft.com/help/4023496/ page.
We understand that you only see 3840 x 1080 there, instead of 4096 x 2304. Please take note that when you connect your Surface device to monitor, the device tries to set the best resolution based on the connector and the resolution of your monitor.
Your aim here is to stretch the capability of your device to show 5K resolution on your gaming monitor. If I may ask, what is the driver version of your display driver when you check Device Manager?
Looking forward for your response.
Kind regards,
Janette_Z