When sending an e-mail, the default PGP key is used regardless of the sender e-mail address present in a different PGP key pairs of keyring.
Symantec Encryption/PGP Desktop always signs messages with the default key, independently of the sender email address used for the outgoing message.
When there are multiple key pairs with different email addresses in the key ring, the key used for signing the message is always the default, regardless of the email address used to send the message.
This product functionality is working as designed. The behavior is similar in standalone and managed clients.
Symantec Encryption Desktop 10.3.x
PGP Desktop 10.2.x