Change of internet provider security code

My internet provider has just switched me to fibre from DSL which meant a new 5G router and a new security router access code. This was registered successfully but each individually on all our Apple devices, iMac, MacBook Pro, iPad and 2 iPhones. We have a visitor with an iPhone, iPad and MacBook Pro with her. All her devices were registered during an earlier visit with the security code for our previous DSL router . We were surprised that after updating the router code separately on her iPhone & iPad, 3 days later when she opened MacBook Pro and expected to register the new code it had apparently already 'learnt' the new router's security code. Should we have been surprised and if not what is the explanation? I am concerned it raises a security issue that I should attend to.