Env: any iOS 15.0+ devices.
I installed the thirtd-party keyboard app like Gboard, Naver SmartBoard, SwiftKey etc. And, I added the keyboard app from Settings -> General -> Keyboard -> Keyboards -> add new keyboard...
However, the third-party keyboard is not shown in safari, finder, but shown in Notes app or the other apps.
I remove the third-party keyboard app from Settings -> ... -> Keyboards and if I add it repeatedly, it is shown.
I think this is a bug of iOS 15. I don't have expreience in iOS 13, iOS 14.