Xiaomi MIUI requires additional setup for notifications to work as intended for apps, including Aqua Mail. To ensure the best possible notification delivery check the following options:
Settings > Notifications > Aqua Mail > Allow notifications
Settings > Notifications > Aqua Mail > Notification categories > Message notifications > Importance > set to High
Settings > Apps > Manage apps > Aqua Mail > App Permissions > Allow all
Settings > Permissions > Other permissions > Aqua Mail > Allow all