According to Nikon, the lens release button "does not function as it should and does not allow for rotation and removal of a lens or mount adapter from the camera", caused by "parts that do not meet our quality standards [being] used on some Z9 cameras".
It's important to note not all Z9s are affected by the issue. Nikon has identified a range of possible faulty cameras and has set up a dedicated support page where Z9 owners can enter their camera's serial number to find out if it requires servicing.
If the serial checker reveals a potentially faulty camera, Nikon has pledged to replace the defective parts at no cost to the owner, even if the camera is out of warranty, and will cover the shipping cost to and from Nikon.
This is a surprising fault, given the lens release button seems like one of the more basic elements of an interchangeable lens camera, but kudos to Nikon for identifying the issue and tackling it head-on. If you are a Z9 user, you best choose which lens you use next with extra care, as you might be stuck with it for some time! A fairly safe bet could be (humor attempt alert) one of the best superzoom lenses - lenses for every eventuality.