Delayed WhatsApp message notification

[k_huat]

My rog phone 5 only alerting arrival of WhatsApp message after I unlock the screen (shown in video). There will be no arrival of messages nor notification at all if the phone is idling in lockscreen. I have emailed myasus and done with all the troubleshooting as advised including factory reset. But the problem still remain. May I know anyone could advise on this issue? Thanks.

[k_huat]

Lord_Messi

https://zentalk.asus.com/en/discussion/comment/187483#Comment_187483

Try to turn off auo restart app

It's works for me

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It got worse if I turn off auto restart. I still get gmail notification if I turn on auto restart; but if turn off, both Gmail n WhatsApp won't get arrival notification. Only WhatsApp with this problem so far (I work using WhatsApp). I tried reinstall it but issue remain.
As long as if I do not open WhatsApp, it stays like that without message arrival notification from time to time, especially if optimized battery, background n sleep mode. Anyhow, thanks for suggestion.