OxygenOS by OnePlus 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 > Apps & notifications > Notifications > Aqua Mail > Battery optimization > Don’t optimize
Settings > Apps & notifications > Notifications > Aqua Mail > Notifications > Enable all desired
Settings > Battery > Battery optimization > (three dots) > Advanced optimization > Disable Deep optimization
Recent apps > Aqua Mail > Tap & hold onto the app window or icon > Lock the app from the options displayed
Guide created with OxygenOS 10 / Android 10. Please note some setting names and locations may differ depending on your OxygenOS version and device model.