Cannot cancel ical event (dreaded "not supported calendar message.ics") - outlook

Question

I'm writing a system in Python that is using the icalendar library to generate ical invites. Those work fantastically, but if I try to send out a cancellation Outlook just displays the dreaded "not supported calendar message.ics". I originally had this issue with the invite, but solved it by fixing my date formatting.
Here is the initial invite:
BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//myapp//myapp.example.com//
METHOD:REQUEST
BEGIN:VEVENT
SUMMARY:Weekly Review Meeting
DTSTART;VALUE=DATE-TIME:20200331T190000Z
DTEND;VALUE=DATE-TIME:20200331T200000Z
DTSTAMP;VALUE=DATE-TIME:20200325T193007Z
UID:1#myapp
SEQUENCE:0
ATTENDEE:me#example.com
CATEGORY:EVENT
CLASS:PUBLIC
DESCRIPTION:bla bla
LOCATION:Webex
ORGANIZER:donotreply#example.com
PRIORITY:5
STATUS:CONFIRMED
BEGIN:VALARM
ACTION:DISPLAY
DESCRIPTION:REMINDER
TRIGGER;RELATED=START:-PT15M
END:VALARM
END:VEVENT
END:VCALENDAR
And then the cancellation:
BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//myapp//myapp.example.com//
METHOD:CANCEL
BEGIN:VEVENT
SUMMARY:Weekly Review Meeting
DTSTART;VALUE=DATE-TIME:20200331T190000Z
DTEND;VALUE=DATE-TIME:20200331T200000Z
DTSTAMP;VALUE=DATE-TIME:20200325T193011Z
UID:1#myapp
SEQUENCE:1
ATTENDEE:me#example.com
CATEGORY:EVENT
CLASS:PUBLIC
DESCRIPTION:
LOCATION:Webex
ORGANIZER:donotreply#example.com
PRIORITY:5
STATUS:CANCELLED
BEGIN:VALARM
ACTION:DISPLAY
DESCRIPTION:REMINDER
TRIGGER;RELATED=START:-PT15M
END:VALARM
END:VEVENT
END:VCALENDAR
The same code generates both and sets the METHOD and STATUS differently (DESCRIPTION also differs). I send a SEQUENCE of 0 for the invite and 1 for the cancellation. The UIDs are always consistent between the matching pair. What am I missing?

Answer 1

I ended up solving this myself. I will share my learnings for posterity.
The MIME type in the email needs to be set with method=CANCEL as well. This was ultimately my issue. method in the MIME type needs to match METHOD in the ical file and both are required for Outlook to process it correctly.
SEQUENCE should not be incremented for cancellation, just for update.
I also updated all addresses (ORGANIZER and ATTENDEE) to include mailto:, but this does not appear to be strictly necessary.