I recently upgraded my VirtualBox from 6.0 to 6.1.18. Auto-resize Guest Display no longer works. I got a black screen if I go full screen. However, if I shrunk to lower resolution, I see the content inside my virtual machine.
I messed around Graphic Controller option but it didn't resolve the issue. Then, when I increased the Video Memory size from 16MB to 32MB, open my virtual machine to full screen work now. I don't know why it requires more memory when the latest version.