MS official statement on this is the XB1 can suspend 4 non-game apps and 1 game.
This seems correct, (I can't remember as the 4th app may close the first), but the important bit being there is a game running which forces those apps to live within the normal "App OS" reservation from a memory perspective. Suspended games stay resident in memory, they just don't have access to the same CPU/GPU resources as they do when they are running (ie. in the foreground). The same is true for apps.
I doubt very much you can have 8 apps suspended.
Not sure if or when this changed, and didn't read exactly what Albert wrote to see if it contradicts what I am saying. But you certainly CAN, so long as there is no game suspended. Sounds like you spent a lot of time investigating this behavior and this is fairly easy to test. Use the menu button to ensure you have quit any game you have open and launch a bunch of apps (more than 5). Then go back to the first app, it will resume rather than reload. This doesn't happen when you have a game suspended. Point being the "App OS" can and will leverage more than 3GB when the "Game OS" is shutdown. And, like you have found, the system will do this automatically after a period of time if the extended recently used app list doesn't include a game.