Using the Update Selected Records button - specifically while viewing a related records list - behaves the way it's supposed to. It lists the fields you can change, lets you change them, and all that stuff. But when you actually apply your changes, no modification is made to the database. The message at the top says that the records were successfully altered, but they weren't changed at all.
The exact same fields can be updated normally in a regular listing. It's just related_record_list that has the issue.