The Android OS is very hostile to background processes running.

The rules around what's okay and what's not is constantly changing, so apps that need to run things in the background are constantly having to update things so they can work properly and sometimes they don't, or they do it wrong, and then certain features don't work properly like push notifications or schedules events.

Then, even if the developer does everything perfectly you get OEMs like Samsung that ignore the rules and fuck with your app anyway, and the literal only way to stop it is to go into the OS setting and disable a lot of things, which puts the onus on the user, and not the developer to make the app work.

But Samsung is such a bitch that even then they don't honor your settings and sometimes revert them, or add new ones that fuck with it again in a constantly changing hard to find nested settings menu.

I actually do like my Samsung phone, but god damn fuck you Samsung for doing this.

If you want a phone that at least follows the ever changing rules properly, get a pixel.