Apps won't close on Huawei MediaPad M5
The applications that are available on the Play Store will not kill Apps as they use the permission GET_TASK which is deprecated from API 26 (Android 5.0) and there is no equivalent for recent Android versions unless if it's a system App (came installed with the phone not from the Play Store).
The solutions to kill the background app is to use an app that came with Huawei system or simply use developers options which can let you kill the background services and processes.
For the second solution, please follow the steps bellow:

Go to Settings &gt; System &gt; About phone on your phone.
Keep tapping Build number until the message &quot;You are already a developer&quot; is displayed.
Go to Settings &gt; System &gt; Developer options &gt; Running services.

You can find a list of third party apps running services. You can click on the App that you want to kill then select &quot;Stop&quot; this will definitely kill the app until you open it again. If the wanted App does not appear in the list you can click on &quot;SHOW CACHED PROCESS&quot; (Top right corner which is not recommended unless you know what are you doing) and it will list all the services and processes.
References:

https://developer.android.com/reference/android/Manifest.permission#GET_TASKS
https://developer.huawei.com/consumer/en/doc/quickapp-open-developer-option