Huawei's EMUI 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 > Notification & status bar> Notification Management > Aqua Mail > Allow notifications

Settings > Apps > Settings (bottom gear icon) > Special access > Ignore optimizations > Allow for Aqua Mail

Settings > Apps > Aqua Mail > Battery > Disable "Close after screen locked"