Starting with ios 12.0 clip board data is automatically erased after a few minutes (after a copy or cut, the data is saved in clip board).
I and others have reported this bug many times ever-since. However, this bug remains in subsequent releases. No comment has been given by Apple. There's no justification for it.
If it's a new privacy feature, It won't help privacy, it's just a nuisance. It definitely needs to be fixed.