Notification doesn't show up on Redmi 8 MIUI 11 and 12
This is usually a problem with battery optimization. Go to your settings and identify the list under battery optimization (that is the stock Android name, it could be battery saver etc). Remove those apps from the list
Edit: As confirmed by OP in comment, it is found under Settings → Battery &amp; performance → App battery saver. Selecting those app(s) from the list, and changing from Battery Saver to No restrictions fixed the problem of missing notifications.