I noticed a strange behavior on my newly-acquired Huawei P8 phone: Notifications about Instant Messages from WhatsApp, Facebook Messenger, and Threema seem to arrive late and in a "batched up" manner. This can take one of two forms:

1. The phone will all of a sudden start to beep and vibrate like crazy, displaying multiple Notifications about new Instant Messages, however, those Messages aren't actually new, they may be anything between several minutes and several hours old, and may come from multiple sources (e.g. a mix of WhatsApp, Facebook, and Threema Messages). E.g. at 11:44 my phone vibrated, and showed me two WhatsApp Messages from two different conversations from 11:33 and 10:02.


2. There will be no Notifications on the phone, and no indicator on the icon, but when I open, say, WhatsApp or Threema, shortly after it has finished loading (less than half a second, usually), several messages will appear at once, both within the app and as Notifications.

It appears as though either the client apps (WhatsApp, Facebook Messenger, Threema) or the Google Cloud Messaging service or both are stopped and/or blocked. This behavior appears both with mobile connections and WiFi, both when on battery power and when fully charged and connected to mains power.

There are various advanced power and data saving options which I am not familiar with from my earlier phones (might be new in Android 6 or Huawei-specific enhancements) which could cause this, but I thought I have everything set up correctly:

• Settings → Airplane mode: OFF


• Settings → Wi-Fi: ON


• Settings → Wi-Fi → Advanced Settings → Keep Wi-Fi on during sleep: Always


• Settings → Data traffic management → Networked apps: both Mobile data and Wi-Fi checked (√) for all System Services, as well as WhatsApp, Facebook Messenger, Threema


• Settings → Data traffic management → Networked apps → Advanced network management: both Mobile data and Wi-Fi checked (√) for all System Services, as well as WhatsApp, Facebook Messenger, Threema


• Settings → More … → Mobile network → Mobile data: ON


• Settings → More … → Mobile network → Enable always-on mobile data: ON


• Settings → Notification panel & status bar → Notification center: Status bar, banners, lock screen enabled for all of WhatsApp, Facebook Messenger, Threema


• Settings → Do not disturb: OFF


• Settings → Accounts → Menu → Auto-sync data: ON


• Settings → Apps → Advanced → App permissions: I did not change anything here and always accepted the default permissions when installing


• Settings → Apps → Advanced → Ignore battery optimizations: Allowed for all of WhatsApp, Facebook Messenger, Threema


• Settings → Advanced settings → Battery manager → Protected apps: ON for all of WhatsApp, Facebook Messenger, Threema


• Settings → Advanced settings → Battery manager → Settings → Close excessively power-intensive apps: OFF


• Settings → Advanced settings → Battery manager → Settings → Keep Wi-Fi on when sleeping: Always


• Settings → Advanced settings → Battery manager → Settings → Keep mobile data on when sleeping: ON

EDIT: A few minutes ago, I received an OTA update (GRA-L09C432B382) with the following description (bold emphasis mine):

This update optimizes the background operation of Google Duo and WhatsApp for an improved experience.

[Optimizations]

• Optimized the background operation of Google Duo, enabling you to make more stable video calls.


• Optimized the background operation of WhatsApp, enabling you to receive new message notifications when your screen is off.


• Optimized the NFC feature by enabling it by default when your device is switched on.


• Provided Wi-Fi coverage in Greenland.


• Expanded the roaming coverage.


• Optimized the display of UI text elements.

[Update notes]

1. If you experience any issues during the update, please call the Huawei customer service hotline or visit an authorized Huawei service center for assistance.


2. This update will not erase your personal data, but we recommend that you back up any important data before updating.

If it turns out that this update fixes the issue, I am going to add a self-answer and accept it.