I installed two apps, PI Music Player and Zello, via apk, vs. via the Google Play Store, in order to get specific features removed in later versions. When I access Battery, Background usage limits, Never auto sleeping apps, and +, these two apps don't appear. In fact, these two apps don't appear under any list under Background usage limits.
You have to add them. When you go to "never sleeping apps", you will get a list of already added apps. Tapping the plus sign on the upper right will bring up a list of all the apps on your phone, and you can select the ones you want to never sleep. If you had already done this step, sometimes new apps don't appear in the list to select right way, you may have to wait several minutes after installing. Also, there is nothing about those apps that can't be put to sleep. They will still work when activated and notifications will still come through.
