Please make sure that on your Android Viewer Settings -> Resolution -> Use native android device resolution is checked.
Then on primary machine side, you can change the secondary display resolution to its native resolution via Display Settings window.
Can you tell us your Xiaomi device’s native screen resolution?
Maybe also send us the screenshot of the spacedesk viewer screen showing the black bar.
For further analysis, could you also please run the spacedesk Driver Console window -> Diagnostics -> click Save All Information, on your primary machine.
PS: Saving all information will take a few seconds, then please send us the whole folder which contains all the diagnostic logs collected.
Please right-click the folder -> Send to -> Compressed (zipped) folder, then attach the *.zip file on your next reply.