iPad Pro with external monitor cannot go full screen

I use a usb-c to hdmi adaptor to connect my iPad Pro 11 inch and an external monitor with 1080p resolution, but I cannot go full screen on my monitor, there is a big black margin around the display area. The same adaptor can connect my Nintendo Switch with full screen without any issue.

In the Apple website it seems this issue can be resolved by setting match content option in settings->display , but there is no such options in my display settings, the OS version is the latest as it is in June 2019(12.3.1) . Appreciate for any suggestions.