I am not sure why you are seeing updates when changing to Italy vs US, but I've had some thoughts on how are labels are formatted on a date time axis that may help us work towards a solution.
The labels on our time axis are formatted using the
labelFormatter property. You could try altering this by assigning a label formatter that uses 12 hour formatting when needed.
A more general soltution may be to attempt to setup a label formatter that uses the users settings. Given that we initialise our formatter with a default NSDateFormatter internally I would have thought that this was already the case, but perhaps the problem is that the original formatter is created with the original 24 hr setting and is not mutated by the user updating their settings, but remains in place on the axis. If that really is the problem then the solution would probably be to create a new formatter each time the user changes their settings (so that the new settings are incorporated into the underlying default NSDateFormatter).
I am not sure if changed user settings are something you can detect in an iOS app - if so you can respond in the way described above. If not then assigning a new date formatter to the axis each time the app becomes active may work (as presumably the app is being exited in order to change the settings and then returned to later).
I hope I've understood your problem properly and that my suggestions are useful! Please don't hesitate to update us on this issue and let us know how you get on with the above info.