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.
Additionally please enable the pop-up windows permissions.
In order to do so please navigate to the MIUI Security app Settings > Apps > Manage apps > Locate the NordPass application and click on it > Other permissions. There, enable the "Display pop-up windows while running in the background" setting.
That's it! The autofill function is now enabled on your Xiaomi device.