Here's a quick guide on how to easily fix it if your current Xiaomi MIUI version prevents alerts from reaching you from various apps.
Open "Settings" from the home screen of your Xiaomi phone and go to the "Apps sub-menu" option.
Find the 'Permissions' option in the Setting menu and click on the 'Autostart' option to go forward.
Choose the apps that aren't sending push notifications and toggle-on the slider to ensure the apps always start in the background.
Use this method if the previous option doesn't work. The work-around ensure apps aren't restricted by MIUI's memory management.
Open the Settings on your Xiaomi device and scroll down to choose Battery & performance option.
Hit "Manage apps battery usage", click on the "Choose apps" option, and select the required app.
When an app is selected, tap "No restrictions" option to turn off 'Doze' feature for the required app.