Encoded-word is apparently used for header line folding sometimes. This appears to me as an abuse of this encoding technique. However, that is not the main issue: it also causes a violation of RFC 2074, as it also encodes message id's:

https://tools.ietf.org/html/rfc2047#section-5 says “An
'encoded-word' MUST NOT appear in any portion of an
'addr-spec'.” and
https://tools.ietf.org/html/rfc5322#section-3.6.4 says
“The message identifier (msg-id) syntax is a limited
version of the addr-spec construct enclosed in the angle
bracket characters, "<" and ">".”

This causes actual problems. Namely, email clients cannot parse the message id and so have trouble with generation of In-Reply-To and References headers or problems with thread reconstruction using these headers containing encoded-word versions of message ids.

Minimal example:

---
>>> import email
>>> import email.policy

>>> msg = email.message_from_string("""From: test@example.com
To: test@example.org
Subject: Test
Date: Mon, 10 Aug 2020 22:52:53 +0000
Message-ID:  <VI1PR09MB41911D8371E899C1FE78EE48FA440@abcdefghijklm.nmopqrst.uvwx.example.com>
X-Some-Blobby-Custom-Header: DIZEglcw6TIh1uC2UrnNjWYqe8l/bYo0oxKG7mBX38s1urzvCwQD30Q07DDJFgTVZWKbThu6hVjR53MTYAHYClHPt8UvyFPkAUIc8Ps1/R+HuSQ8gbR1R03sKoFAgPZKO+FKJ9bNbBb60THl81zSCsZiALwi4LLOqnf9ZIB111G4/shFuWxRlPcsPJt72sn+tTHZqK9fRAyoK1OZCZMJmjQGysovicz1Xc6nOXHMQr2+suRwOJwSUqvsfkj8EEtzJGj7ICQ2GbgBaOjcof1AML4RCFy/vD5bG0Y8HQ2KET3SraTki4dPo+xMYSZVFEy/va4rYeynOXPfxXfHSyIFwB6gnH74Ws/XPk8ZxhAQ2wSy7Hvgg3tZ7HOmlLWg4A/vUGN+8RJlgn+hHtuCXnglv+fIKEhW36wcFotngSrcXULbTlqdE5zjuV5O7wNfgIShZnNhnPdLipslmZJGaa6RQpIonZbwUWCM8g9DZmSwo8g0On0l20IVS9s6bUCddwRZ5erHx4eUZ4DGh4YyR2fgm0WsNVW8pVsAdFMClfAJYqyPEqrDN91djfPYRZPMvzYWTAm8MAip6vDa1ZvzywDpGJYD3VwapLfgFy+AR0S/q/V1HHRmSXx1oNLEedhAt0OkIxWxO8FvqNeEfMLVhxTk1g==
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Content-Type: text/plain; charset="utf-8"

BODY
""")

>>> print(msg.as_bytes(policy=email.policy.SMTPUTF8).decode())
From: test@example.com
To: test@example.org
Subject: Test
Date: Mon, 10 Aug 2020 22:52:53 +0000
Message-ID: =?utf-8?q?=3CVI1PR09MB41911D8371E899C1FE78EE48FA440=40abcdefghij?=
 =?utf-8?q?klm=2Enmopqrst=2Euvwx=2Eexample=2Ecom=3E?=
X-Some-Blobby-Custom-Header: =?utf-8?q?DIZEglcw6TIh1uC2UrnNjWYqe8l/bYo0oxKG7?=
 =?utf-8?q?mBX38s1urzvCwQD30Q07DDJFgTVZWKbThu6hVjR53MTYAHYClHPt8UvyFPkAUIc8P?=
 =?utf-8?q?s1/R+HuSQ8gbR1R03sKoFAgPZKO+FKJ9bNbBb60THl81zSCsZiALwi4LLOqnf9ZIB?=
 =?utf-8?q?111G4/shFuWxRlPcsPJt72sn+tTHZqK9fRAyoK1OZCZMJmjQGysovicz1Xc6nOXHM?=
 =?utf-8?q?Qr2+suRwOJwSUqvsfkj8EEtzJGj7ICQ2GbgBaOjcof1AML4RCFy/vD5bG0Y8HQ2KE?=
 =?utf-8?q?T3SraTki4dPo+xMYSZVFEy/va4rYeynOXPfxXfHSyIFwB6gnH74Ws/XPk8ZxhAQ2w?=
 =?utf-8?q?Sy7Hvgg3tZ7HOmlLWg4A/vUGN+8RJlgn+hHtuCXnglv+fIKEhW36wcFotngSrcXUL?=
 =?utf-8?q?bTlqdE5zjuV5O7wNfgIShZnNhnPdLipslmZJGaa6RQpIonZbwUWCM8g9DZmSwo8g0?=
 =?utf-8?q?On0l20IVS9s6bUCddwRZ5erHx4eUZ4DGh4YyR2fgm0WsNVW8pVsAdFMClfAJYqyPE?=
 =?utf-8?q?qrDN91djfPYRZPMvzYWTAm8MAip6vDa1ZvzywDpGJYD3VwapLfgFy+AR0S/q/V1HH?=
 =?utf-8?q?RmSXx1oNLEedhAt0OkIxWxO8FvqNeEfMLVhxTk1g=3D=3D?=
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Content-Type: text/plain; charset="utf-8"

BODY
---