Why is the application icon in the dock greyed out?

Question

iOS

I updated my old iPad Air to iOS 11. I noticed that sometimes some icons of recently launched applications in the dock become dimmed and do not respond to pressing (see screenshot). At the same time, these same applications can be opened from the desktop / control center without any problems. After removing the icon from the dock and restarting the application, the newly appeared icon becomes active. But after a while it (or some other) becomes gray again. I can't figure out what mechanism this has, what it means, and do I need to do something about it if this happens to your application? Couldn't google anything on the subject.