I ran into the exact same issue here, when an ability system component (ASC) was added in C++ to the project’s base Character class. When an existing Character subclass that derived from the project’s base Character was opened in the editor, the component would exist in the ‘Components’ tab however the details would not populate under the Details tab when the component was selected, and in-game the component would be null.
Similarly to what Babymaster3000 posted, I reparented the Character subclass to the default Character (ACharacter) class, and then re-reparented the Character subclass back to the project’s base Character class. After the reparent the ASC populated details of the Details tab and the in-game value of the component was valid.