.ICS calendar item works in Outlook desktop client, but not OWA webmail

Question

I have an app that allows users within our organization to register for and drop classes available to the staff. Both of these actions generate a confirmation email and I've added .ICS calendar items to the emails, one to add the class to the registrant's calendar, and the other to remove it.

Both of these work exactly as expected in Outlook, but the "METHOD: CANCEL" one fails in Outlook Web Access with error message: "The .ICS attachment can't be viewed because it contains an item that's not calendar-related." I've tried downloading the files from desktop and webmail and comparing them, and they are identical, so no issue there.

Using Microsoft Office LTSC Professional Plus 2021

Here is the "Cancel" .ics file:

BEGIN:VCALENDAR
PRODID:-//Microsoft Corporation//Outlook 16.0 MIMEDIR//EN
VERSION:2.0
METHOD:CANCEL
BEGIN:VTIMEZONE
TZID:Eastern Standard Time
BEGIN:STANDARD
DTSTART:16011104T020000
RRULE:FREQ=YEARLY;BYDAY=1SU;BYMONTH=11
TZOFFSETFROM:-0400
TZOFFSETTO:-0500
END:STANDARD
BEGIN:DAYLIGHT
DTSTART:16010311T020000
RRULE:FREQ=YEARLY;BYDAY=2SU;BYMONTH=3
TZOFFSETFROM:-0500
TZOFFSETTO:-0400
END:DAYLIGHT
END:VTIMEZONE
BEGIN:VEVENT
DESCRIPTION:Training class XYZ
DTSTAMP:20240311T092043
DTSTART;TZID="Eastern Standard Time":20240313T110000
DTEND;TZID="Eastern Standard Time":20240313T120000
SEQUENCE:0
SUMMARY;LANGUAGE=en-us:Training Class XYZ
LOCATION:Room 151
TRANSP:OPAQUE
UID:69EEED22-E111-436C-9210-6B95000A55D5
END:VEVENT
END:VCALENDAR 

...and for good measure, here is the "Publish" .ics file. The only difference between them is Method: PUBLISH vs CANCEL and the timestamp. They both have the same GUID.

BEGIN:VCALENDAR
PRODID:-//Microsoft Corporation//Outlook 16.0 MIMEDIR//EN
VERSION:2.0
METHOD:PUBLISH
BEGIN:VTIMEZONE
TZID:Eastern Standard Time
BEGIN:STANDARD
DTSTART:16011104T020000
RRULE:FREQ=YEARLY;BYDAY=1SU;BYMONTH=11
TZOFFSETFROM:-0400
TZOFFSETTO:-0500
END:STANDARD
BEGIN:DAYLIGHT
DTSTART:16010311T020000
RRULE:FREQ=YEARLY;BYDAY=2SU;BYMONTH=3
TZOFFSETFROM:-0500
TZOFFSETTO:-0400
END:DAYLIGHT
END:VTIMEZONE
BEGIN:VEVENT
DESCRIPTION:Training class XYZ
DTSTAMP:20240311T091944
DTSTART;TZID="Eastern Standard Time":20240313T110000
DTEND;TZID="Eastern Standard Time":20240313T120000
SEQUENCE:0
SUMMARY;LANGUAGE=en-us:Training Class XYZ
LOCATION:Room 151
TRANSP:OPAQUE
UID:69EEED22-E111-436C-9210-6B95000A55D5
END:VEVENT
END:VCALENDAR 

Does anyone have any suggestions?