I think what you're seeing is that not all databases have foreign keys - some applications hard-code the relationships vs letting the database manage that.
Log in to that different oracle instance, and browse the affected tables - do they have FK constraints defined on them?
Thank you very much for your help and your attention to my problem. Much appreciated.
Sorry for the delay I was waiting for the developer to have the confirmation.
She said that indeed this set of tables were created just by a copy of tables (create table as (select . . . ))
and therefore the keys and constraints were not created on the copy version & I didn't know that.
Thank you very much for your remark & your help. It's clear now what was the problem.