Ive had this today from Nuki…….
“Recently, there was some issues with Matter integrations connection and configuration, however, this issue does not come from our side or from our device but is due to Apple iOS 18.4, Apple confirmed that the issue should get solved with the release of their next iOS update, however you may ask Apple to give you access to the current iOS beta to check if this solved your issue. You may want to check the follow article on our website to know more about the issue”
Seems its a confirmed apple issue