In Zabbix 5.4.9 we've configured special characters in the Subject of emails. After upgrade to 5.4.10 this remained working. However after having upgraded to Zabbix 6.0.0 these special characters are now replaced by '??'

Steps to reproduce:

1. Go to Administration - Media Types - Email - Message templates.
2. Configure a template (for example the problem template) and use a special character. like '⚠️'.
   #Configured template:

Expected:
That the special characters in the subject are shown, or that it's clear how they can be shown.

Result:

Result in Zabbix 5.4: The special characters are shown in the email subject

Email header related lines::

Subject: =?UTF-8?Q?WSEMon=E2=9A=A0=EF=B8=8FInformation:_Version_of_?=
	=?UTF-8?Q?zabbix=5Fagent=28d=29_was_changed_on_Z?=
	=?UTF-8?Q?abbixserver?=
MIME-Version: 1.0
Content-Type: multipart/alternative;
	boundary="----=_NextPart_000_0093_01D82448.7C28E610"

Result in Zabbix 6.0.0: The special characters in the email subject are replaced with '??'

Email header related lines:

Subject: WSEMon??Updated: Version of zabbix_agent(d) was changed on Zabbixserver
MIME-Version: 1.0
Content-Type: multipart/alternative;
	boundary="----=_NextPart_000_008C_01D82448.7B3648B0"

It looks like the special character is not translated to ?UTF-8?Q? code.

Remarkable extra info:

• When using the email TEST, the special characters are nicely shown.

  Subject: =?UTF-8?B?VGVzdCBzdWJqZWN0IE1vbuKaoO+4jw==?=
  MIME-Version: 1.0
  Content-Type: text/html; charset="UTF-8"
  Content-Transfer-Encoding: base64
  

• When I replaced the content of the subject field with the result of the working 5.4 (so adding ?UTF-8?Q?.....?= it's still not working.
  

  Subject: WSEMon?Resolved: Apache: has been restarted (uptime < 10m)
  Date: Sat, 19 Feb 2022 16:42:21 +0100
  Message-ID: <[email protected]>
  MIME-Version: 1.0
  Content-Type: multipart/alternative;
  	boundary="----=_NextPart_000_0262_01D825AF.C7456CF0"