If you check your email with Comcast's Xfinity Connect webmail, look for the XFINITY Verified Email logo next to any message that claims to be from Comcast/Xfinity (see https://www.xfinity.com/support/articles/comcast-verified-email). If the message is genuine you'll need to have a Comcast rep cancel the cancellation.
If @ComcastCSAEmail , @ComcastTeds or another employee doesn't reply to your message here, Comcast Security Assurance might be able to help. You can reach them at 1-888-565-4329 (from https://internetsecurity.xfinity.com/help/report-abuse/). Note that this is NOT one of the general customer service numbers.
The first-line reps there are sometimes not very helpful, so you may need to be persistent and keep at them until you get a satisfactory answer.
See https://forums.xfinity.com/t5/Customer-Service/Email-saying-quot-we-have-received-your-cancellation-... for a similar situation.