When you create issues via email, the subject of the created issue may be truncated if the subject field in the original email is split into multiple lines in singlebytes and multibytes mixed format like the following.

To: redmine@somenet.foo
From: John Smith 
Subject: =?UTF-8?B?W+e3iuaApV1bUkVETUlORV1bV0FSTklORzAxXVByb2JsZW06IOODoQ==?=
 =?UTF-8?B?44Oi44Oq5a656YeP44GM6YC86L+r44GX44Gm44GE44G+44GZ?=
Message-ID: <42feb4b3-4dec-7c61-0edd-45dfc4795143@somenet.foo>
Date: Tue, 18 Jun 2019 14:08:06 +0900
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 7bit

Problem started at 13:49:27 on 2019.06.18

The Subject should be decoded like this:

[緊急][REDMINE][WARNING01]Problem: メモリ容量が逼迫しています

But the subject of the created issue will be the following:

[緊急][REDMINE][WARNING01]Problem: メ

Attached a test.