You could disable some dates even on client side.
isDayEnabled attribute could be pointed to some js function which will get the date as a parameter and return boolean value which means if day enabled. This function called by the calendar during dates initialization for every date from the month.
Switching monthes back - could be optionally turned off at all while you at current month using cancelable event oncurrentdatechange.
If you stil want to use model - just check your values in debug after switching month back. Seems there are just wrong disabled property value for the dates before current month.