Update
The problem re-emerged after an OS update. This time my partner was told she needed to accept the T &Cs on all Apple devices. Not a very helpful demand!
Eventually I found that the issue was to do with the Homepods being default audio output for an Apple TV. Every user of the ATV needs to switch to their profile then launch the Music app. After accepting the splash screens on the ATV the Homepods should start to obey requests from that person.