@mention and general update emails from batch notification are not grouped together, despite having the same email subject.

Summary

@mention and general update emails from batch notification are not grouped together, despite having the same email subject.
The issue only happen to email with double-byte characters in the subject.

How to Replicate

1. Enable batching email notifications
2. Create an issue with summary in Korean characters
3. Add a regular comment to the issue
4. Add a comment with @mention in the issue
5. Check the emails from mail client

Investigation Done

• We noticed the email grouping problem with MS Outlook and Thunderbird. Problem is not affecting the grouping in MacOS Mail client.
• From the emails that we received from batch notifications, we can confirm that the subject line looks identical from the email client.
• Reviewing the email headers, we noticed the differences in the email subject in encoded format (though they decoded to the same value):
  • @mention email subject

     Subject: =?UTF-8?Q?[Test_JIRA_-_Ignore_it]_Upd?= =?UTF-8?Q?ates_for_WT-1?=
     =?UTF-8?Q?1:_=EB=A1=9C=EC=A7=81_=EC=88=98?=
     =?UTF-8?Q?=EC=A0=95_=ED=85=8C=EC=8A=A4=ED=8A=B8_4?=

• • General notification email subject:

     Subject: =?UTF-8?Q?[Test_JIRA_-_Ignore_it]_Upd?= =?UTF-8?Q?ates_for_WT-11?=
     =?UTF-8?Q?:_=EB=A1=9C=EC=A7=81_=EC=88=98=EC=A0=95?=
     =?UTF-8?Q?_=ED=85=8C=EC=8A=A4=ED=8A=B8_4_?=

• The header from email subject in English:

   Subject: [Test JIRA - Ignore it] Updates for WT-10: As a developer, I can
   update story and task status with drag and drop (click the triangle at far
   left of this story to show sub-tasks) 

Please Help With

Please let us know if we can standardise the encoded email subject or encode it one-go, instead of doing it in chunks e.g:

 Subject: 
 =?UTF-8?Q?[Test_JIRA_-_Ignore_it]_(WT-11)_=EB=A1=9C=EC=A7=81_=EC=88=98=EC=A0=95_=ED=85=8C=EC=8A=A4=ED=8A=B8_4?=

Workaround

N/A