To save the RAM, apps are usually being removed from the Recent Apps list, but with the removal of apps from the Recent Apps list, the app is closed and some times important applications like WhatsApp, Facebook, etc are also removed and killed by android.
In order to make those apps or Smart Messenger and Self Reminder app from being killed, the user needs to add the apps to whitelisted apps.
Different mobile manufacturers have settings for adding apps to whitelisted apps.
Like in Xiaomi phones, the user needs to Lock the app in the Recent Apps List and Set the Battery Saver for that particular app (Settings > Apps > Smart Messenger with Self Reminders > Battery Saver > No Restrictions).
Along with this, also do the following steps:
Go to Settings > Apps > Smart Messenger with Self Reminders >
Click AUTO START and scroll down and select Battery Saver.
Then Select NO RESTRICTIONS.
Now, the app won’t get removed and users can get important reminders on their scheduled timings.
See Also: App does not show Alarm or does not work in background