Hi hello-hihi,
Thank you for reaching out Microsoft Community. Sorry for the delayed response.
To initially troubleshoot this issue, I'd like you to clear the display cache first. Please follow Solution # 5 from this link>>https://support.microsoft.com/en-us/help/4023496/surface-troubleshoot-connecting-to-a-second-screen. After clearing the display cache, kindly disconnect the monitor and the Surface device, restart your Surface and re-establish its connection again. Once done, you can press and hold Windows + P and choose "extend". If the second monitor still has black borders on the top and below screen, check in Display Settings the recommend resolution intended for the second display.
If this helps, feel free to mark this post as an answer. Otherwise, don't hesitate to respond to this thread if you need further assistance.
Kind Regards,
Crest_Z