Notification email not sent out when "Assign responsibility >> Responsible user" selected

Problem

• Notification are not being sent out when "Assign responsibility >> Responsible user" is added

Replicate

1. enable DEBUG
   <bamboo-install>/bin/setenv.sh

   JVM_SUPPORT_RECOMMENDED_ARGS=-Dmail.debug=true
   

   <bamboo-install>/atlassian-bamboo/WEB-INF/classes/log4j.properties

   ## Troubleshooting Notifications (Email & IM)
   # https://confluence.atlassian.com/bamkb/troubleshooting-notifications-email-im-216957451.html
   log4j.logger.com.sun.mail=DEBUG
   log4j.logger.com.atlassian.bamboo.mail=DEBUG
   

2. create Project / Plan
3. have two user in Bamboo

   username	email	author
   admin	admin@domain.com	admin
   committer	committe@domain.com	committer

4. commit something with user 'committer' making the build to fail
5. add 'admin' as "Assign responsibility >> Responsible user"
6. commit something with user 'committer' making the build to succeed
     Notification is not sent out

Tests executed on

• Bamboo v5.9.3
• Bamboo v5.10.3

Notes

• just to make sure emails were sent out, Notification was changed to "All Builds Completed" and 'admin' received email.