The Hide My Email security feature doesn't always work properly?

Hi everyone 😊

I used the Hide My Email feature in Safari browser at a merchant with an online store to register. I expected an incoming email with payment information from the merchant. But none came.

I verified with the merchant by phone that they sent payment instructions to the email address generated by the Hide My Email feature. The merchant confirmed that the email was sent. I asked the merchant to try sending the email again manually during the ongoing phone call. The merchant confirmed that he was resending to the email generated by the Hide My Email feature. None came.

I dictated my real (not hidden) email address to the merchant to send it repeatedly to during the ongoing phone conversation. The email with payment instructions from the merchant came immediately and twice. Once with my (unhidden) email address and at the same time incoming mail to the same email thread with the hidden address from the Hide My Email feature.

What's wrong?

macOS 15.4.1 (24E263)

Safari 18.4 (20621.1.15.11.10)

Mail 16.0 (3826.500.181.1.5)