this is a general xiaomi problem because of their aggressive ram management. currently im struggling with notifications from element app. i switched back to stock rom and theres the same problem. none of the settings (battery restriction, autostart, app pinning and so on) are working. sometimes the notification comes with a hughe delay and sometimes only if i unlock the device and sometimes they only come when i open the app. otherwise some apps like threema or nine mail client are working fine. this is really annoying and not an xiaomi.eu custom rom problem its made by xiaomi itself. but i've read that there should be improvments in this issue with miui 14.