Zone designations are account specific; zone D for you is not zone D for me
The affected AZ appears to be use1-az6. You can map "your" AZ name (us-east-1c, us-east-1d, etc.) to the actual, canonical name of the AZ in the 'Subnets' tab on the VPC console.
What makes you say use1-az6 is the culprit? I only ask because none of our workloads in az6 have experienced any issues. ....yet. We run critical workloads across 3 AZs thankfully, but still.
What's the best way of figuring out which zone it could be in my account?
Running
aws ec2 describe-availability-zones --region us-east-1 --output text