Currently when an external sender sends a meeting invitation (may be from Outlook) to a Notes Client user and if sender cancels the meeting and then goes to Trash and restores that meeting in Outlook and then reschedules it, then Notes Client user only receives "Reschedule" notice but there are no "Accept" or "Decline" buttons in that meeting update. So there is no way for Notes Client user to "Accept" or "Decline" that reschedule.
Once the meeting is cancelled then Notes removes it from Calendar. After that any update received on that meeting cannot be processed by user since original meeting document is no longer present in the calendar.
This is working as designed at this time. Notes is designed to remove the meeting from calendar once it is cancelled. It cannot process any update notice thereafter for the same meeting.
So there is a feature request to allow Notes to restore the meeting in Calendar so that update notice can be processed by user with "Accept" or "Decline" buttons.
The only workaround to this issue is to go to "All Documents" in Notes and then find the required meeting and its update notice. Open the update notice to view the latest time of the meeting. However this meeting cannot be viewed in Calendar.
This Enhancement Request was submitted for Notes Client V11 version.