Checkout has its own advance-notice check separate from the calendar. If the site-wide HomeRunner setting ‘Validate advance booking hour’ is set to a non-zero value, checkout uses that number of hours for every property regardless of the property’s synced PMS value. Only when that setting is 0 does checkout fall back to the property’s synced value. This means the calendar can grey out more days than checkout actually rejects if the two settings are mismatched. Make sure ‘Validate advance booking hour’ is set to the same number of hours as your intended window so the restriction is enforced end to end.