I verified that the ICS contents are valid using a couple of online validators, and it passed validation.

I tried all the methods mentioned here:
https://stackoverflow.com/questions/27662899/outlook-365-receives-a-message-that-has-an-attachment-that-is-named-not-support

But nothing helped.

Currently, my suspicion is that this happened because I used a SendGrid account to send the event cancellation, which may have been detected as a sender other than the organizer.

I should test this on a new event created directly from a non-SendGrid account.

The same problem still occurs even when sending from a Gmail account, and when sending from a personal Microsoft account.

I'm currently at a loss. I have no idea why this is happening.

Not sure if you are still supporting this, but I found that changing the name of the ics file from Meeting.ics to invite.ics caused the cancel mail to work.

Wait, seriously? That easy? I'm gonna try this myself ASAP

Not sure if you are still supporting this, but I found that changing the name of the ics file from Meeting.ics to invite.ics caused the cancel mail to work.

I tried making the change but I'm still getting the same issue with the "not supported calendar message.ics" attachment.

I pushed the changes to the v2.4 branch. Are you sure about this solution?

You are correct. I was getting it to work when I was sending the invite attachment from gmail. If I send it from the SQL server to my outlook account, it is giving the same error. I will keep looking into it.

I spent a few more hours looking into this, and am unable to figure out why it isn't working. The work around that I came up with was to send an update meeting request in the distant past and set a mailbox rule to auto delete events older than x number of days. I know this won't work for everyone, but it may help some.

For reference, I generated a calendar invite using the native Outlook web-based interface and then cancelled it.
I captured the attached invite.ics file that arrived with the cancellation, and I'm adding it here:
invite.zip

I don't have the time to do it right now, but I should compare this file's contents to the contents generated by my CLR method, and take note of the differences. Maybe I'll find something.