Hello,
To reproduce this, create a new project, add an actor blueprint (Parent), right click and “create child blueprint class” (Child) then right click on the child actor and select “duplicate” (Child_Duplicate).
Now create another actor blueprint (RandomActor) unrelated to the previous 3 and inside add a class Parent type variable and make it public.
Add an instance of RandomActor in the scene and set the variable value to class Child or Child_Duplicate. While doing so you will notice that all 3 classes are visible in the dropdown.
Close the engine and reopen, go to the dropdown and you will notice that a child class is missing from the dropdown.