Likely reason is battery saving feature of Huawei which is a stronger version of doze-mode, in that apps are frozen if inactive after a duration. I had seen this on my earlier Huawei device. See my answer here. What is protected apps in Huawei phones?.
It looks like it is not called "protected apps" anymore and this Reddit explains the required settings to disable this for apps. There is also a feature of Power Genius involved and a workaround to bypass that. Options mentioned in Reddit
See if you can find a way to turn off battery savings in this Location or similar Settings- apps & notifications - apps- settings cog- special access - ignore battery optimisation >allow/disallow
Settings -> Apps -> Apps -> three dots on upper right corner -> special access -> battery optimization -> change dropdown menu to "all apps" -> change apps you want to keep in background to "don't allow, may drain battery more quickly
Edit: OP confirmed that the second option was the right one for their device. Thanks
Disable battery saving feature for those apps and hopefully that should fix delayed notifications.