This was initially reported as a KMail bug: https://bugs.kde.org/show_bug.cgi?id=327986
IMO the issue here is: If you get a message for hidden recipients you are asked to unlock every secret key you have. Even if the session key could already be decrypted with one of them.
I can reproduce it by:
- Encrypt a message to hidden recipients including a key that is already unlocked / passphrase entered.
- Decrypt the message.
Result is that I'm asked for the passphrase of all my keys.
IMO the priority here is Whishlist because hidden recipients is an expert / non standard feature for which we don't need good usability but might wish to improve usability for.