In order to allow the NordPass Autofill Services on your Xiaomi phone, you need to enable additional permissions for it to properly work.
To do that, go to the MIUI Security app Settings > Permissions > Autostart. There, select NordPass.
Alternatively, open up the NordPass application settings on your device, go to Other Permissions and enable Start in background and Show pop up permissions.
That's it! The autofill function is now enabled on your Xiaomi device.