Why the app icon in the dock inactive?

Updated my old iPad Air to iOS 11. Noticed that sometimes some icons recently running apps in the dock be dimmed and does not respond to clicks (see screenshot). At the same time, these same applications without problems open from the desktop/control center. After removing icons from the dock and restarting the application, new icon will become active. But after some time she (or any other) again becomes gray. Can't understand what this mechanism is, what it means, and whether this is something to do if this happens with your app? Googling anything on the subject could not.
June 14th 19 at 20:12

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