I think iOS basicially closes an app when you leave it, and that’s partially the reason they have such good battery life.
If you have a custom ROM like graphene, or Volla’s ROM I think by default the settings are similar to iPhone where basicially nothing runs in the background and doesnt constantly ping home to google. You have to dig in the settings and let it happen.
If you have a low spec phone that might be another reason it closes it out when doing another task.
I can only assume thats just the nature of it then.