Android Lollipop introduced screen pinning: a way for you to lock your device into one app until a specific shortcut was tapped to take you out of it and let you switch to something else.
In Lollipop, a screen is pinned by going to Recents and tapping the green pin button at the bottom right of any app card, and it is unpinned in one of two ways: short tapping Recents and Back simultaneously if no Accessibility service is enabled in your Settings at all, or long tapping Recents if at least one Accessibility service is switched on. That created several problems:
- the confusion over which shortcut to use depending on whether you have some Accessibility service enabled,
- the automatic switch to Recents each time you unpinned (you were pressing Recents after all...),