I'm also experiencing the same issue. Using Apple iOS Mail and receive the encrypted mail message but unable to decrypt the message unless I install the profile. If I install the profile (via .p12 that includes the public and private) then the email message can be decrypted just fine. However, the issue is that I don't want and cannot put the .12 (or the private key) on the device. I'm also using cryptotokenkit and can sign with the private key that exists on my YubiKey.