In a Microsoft Skype for Business Server 2015 environment, you create a Skype for Business mobility policy in which the AllowSaveCredentials parameter is set to False. In this scenario, users can't save their credentials information (such as passwords) on their mobile devices. However, when a user tries to sign in to Skype for Business by using the Skype for Business on Mac desktop client, the password is also not saved even if the user selects the Keep me signed in option on the client. It's expected that the Skype for Business on Mac desktop client can save the sign-in credentials.
The issue occurs because the mobility policy was applied to the Skype for Business on Mac desktop client, and it was by design. After you apply this update, the mobility policy is applied only to the mobile clients.