CiviMail: What is the expected behaviour when unsubscribing from a mailing sent to a parent group?
We abandon this method of manage mailing lists and turn on adding members to mailing groups as "expected" (not with parent or with smart groups). GDPR regulations pushed us to get this direction and avoid the problems described in your question at the same time.

Maybe we should complete the documentation to advise against the use of parent groups as a mailing groups, as long as it is possible to do so.