The viewport configuration UI in the global Confluence administration is not able to completely save a viewport if the current user is 'only' an application or system administrator but lacks space admin permissions in the space the viewport belongs to.
The viewport itself can be saved, but the settings from the viewport permissions tab (shared among all viewports in a space) cannot be saved in this case.
Since global administrators can always grant themselves admin permissions in a space, and they can already save the viewport configuration, the safety check for managing viewport permissions should be 'relaxed' so that either space or application administrators can perform these operations.