Hi!
As I remember the ages before CT999, SFC Split devided the existing CT data between the parent and the child SFCs proportionally, by default. And, I guess, to make this behavior configurable, CT999 was introduced. Thus, I believe that BOM should be inherited by a child SFC by default once the child remains to belong to the same Shop Order which defines the BOM of the parent SFC at release, however, whether the child inherits collected assembly data of the parent or not should be controlled by CT999.
In later releases there was a fix of SAP Note 2164161 which might be relevant to this issue. So, it would be good to test it there.
Regards,
Sergiy