Thank you for reaching out with this question. I looked at the mailing and did a bit of math. The actual percentage of opt outs from your mailing equals 0.422%. Because of this, Emma rounds the number down to zero. If it had been at least .5% you would have seen Response show 1% for opt outs on this mailing. Make sense?
Let me know if you have any lingering questions about this! Happy to help.