iOS just keeps getting worse. When I install new apps I cannot find them so that I can move them into the appropriate app group. I’ve looked in the recently installed group, they are not there. The icons are not visible on any of the desktops.
I’ve seen this issue occurring as well. The “smart” groups in the app library is not that smart at all. They’re not appearing in the recent group until device restart. But you can search for the new app in spotlight or find it in the alphabetical list by pulling down in the app library.
Under Settings > Home Screen you can say if a new app should go to the App Library or Home Screen which may help.