It seems on EMUI 9+ (Android P+) Huawei introduced a new task killer app called PowerGenie which kills everything not whitelisted form Huawei and does not give users any configuration options. See http://dontkillmyapp.com how to uninstall it and other tips.
Also you can try to temporary disable it:
We did not yet had this confirmed but it is possible you can alternatively just disable PowerGenie in Phone settings > Apps but this setting needs to be re-applied every time you reboot your device.
Also another user has reported that this settings has solved the problems on Huawei:
I've made adjustments to the following, via easily accessible apps:
1) As previously mentioned, on the Huawei Phone Manager app settings, I've switched off "Auto Smart Tune Up". This claims to "regularly update app configuration to improve app performance".
2) Via the Battery app>app launcher, I switched off "Manage All Automatically". I've gone through each app I can access and put in my own settings.
3) I've done a similar exercise to 2) above with "notification" settings.
By having completed action 1), the settings in 2) and 3) seem not to be altered by the system, and I'm now satisfied with the phone operation and battery usage. Whether this would work for others I don't know.