Why does a MIMEMultipart email introduce a trailing newline when viewed as text message?

​The following MIMEMultipart/mixed email message was sent from a gmail account to an iOS recipient via mms.att.net. The message includes a MIMEText subpart and a MIMEImage attachment. Why does the text bubble include a trailing newline?​

​The email contents:​

Received: from 116692035092 named unknown by gmailapi.google.com with HTTPREST; Sat, 25 Mar 2023 18:47:14 -0700
Content-Type: multipart/mixed; boundary="===============6546997771072418005=="
MIME-Version: 1.0
to: [email scrubbed]
from: [email scrubbed]
subject: 
Date: Sat, 25 Mar 2023 18:47:14 -0700
Message-Id: <CAJLYKcrfN4QNYikE85rvV2A1V00=[email scrubbed]>

--===============6546997771072418005==
Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit

Hello!
--===============6546997771072418005==
Content-Type: image/jpeg; name="27e0768e981cb35c.jpg"
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="27e0768e981cb35c.jpg"

--===============6546997771072418005==--

​The Message screenshot shows an extra newline at the end of the text bubble.​
​Message screenshot​

​This renders correctly when sending only a single subpart, either MIMEText or MIMEImage. This appears to be related to how Messages handle MIMEMultipart/mixed types, or due to a bug introduced in the MMS gateway. The recipient is iOS 16.3.1.​