Frustrated by my inability to unlink some text boxes, (which I initially blamed on some strange InDesign behaviour I was experiencing) it suddenly dawned on me that the job had been set up with linked Facing Pages masters text boxes. I'm not sure I ever realised that unless you break the Master page link 'Master>Detach all objects from Master' the links remain and it's impossible on those master page items to unlink whilst the connection remains. All these years... who knew?
Sorry for the delayed response. This is an expected behavior with all Master page items. For any edits on master page items you will have to Override or detach master items.
For more details, please take a look at this help article. Please feel free to reach out if you run into any other questions or issues in the future.