Images not showing up in email

I have a small python script that emails out a comic strip once a day to a handful of email addresses. Recently, one person on my list told me that the images are no longer displayed. I got the source of the email from her Gmail account, and for some reason the base-64 encoded section for the image in blank.

I have no idea why this is happening, and why it's happening to only one person. I get an SPF soft-fail in the Gmail headers, but I doubt that's causing it (that should happen to all the Gmail users on my list.) I don't even know what to Google to start troubleshooting this issue.

Email Source {censored items in curly braces}:

Delivered-To: {recipient}@gmail.com
Received: by 2002:ac2:555e:0:0:0:0:0 with SMTP id l30csp2647742lfk;
        Sun, 22 Mar 2020 02:12:09 -0700 (PDT)
X-Google-Smtp-Source: ADFU+vt1SopHnbnPh0tuHjbA/v3RMASeIKMyk35ZdIXqQHBr2LmtG11vVo6cI8XXxUf/emCJQjqb
X-Received: by 2002:a92:25d6:: with SMTP id l205mr14876228ill.35.1584868329096;
        Sun, 22 Mar 2020 02:12:09 -0700 (PDT)
ARC-Seal: i=1; a=rsa-sha256; t=1584868329; cv=none;
        d=google.com; s=arc-20160816;
        b=o1VVDgxlMMWGRPy041qJS6LfA1LWjEB6+cbp768sLmMaOk7Q+qbCpAg0Mydf3pg/yu
         N3Nfh0Zv7vUdS6sapH0LzmcCG6YVBC/fkGboFuUeVJgVw24CcY8e1ofkMmIkWardIQie
         bNfAnebSMmIhmZ6UTrR79ggRhUglFx6EdAzNYdmpyRCxewK3/SN5JjdfW6KUQsgjQXU5
         PvJQEZZrt07eKjzq2kpNVobWBj8rOvVGpYc+fqNDOircTIF5ZTI3JIbCEkNvvEyAmZOW
         AK68xVtSpaYfInnJ0++dPkS9DhuH777f/yMz6Z0X26MjnoxiIovyYSNPfzZLdjpnC/CI
         G4nQ==
ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816;
        h=date:message-id:to:from:subject:mime-version;
        bh=nXPNVg7/Z2G2UDLCbHAibUHl0oCh4sJ6yhHsm9HpP+Y=;
        b=Vl5At9J4nLzeREzg93sHypB2XO/vbNHAtRj4apOzQZHLg0XbjD3Hxv6t9+yzFn8thS
         pFnPRPCOOgyY+mp+8hTQrT5TaIlEcNC9Ay1x25X27wqcr0iBjw9xpTvjvFo1pX7jNK7E
         DvUZpdSoSmas32U0zT8DH1+jX5zX/8Ydr7aTOkVLPX6umVuyl2RNlW+mLZrPnCMkiH0Z
         jeTc88LMiczwsVwo2qsoKPDjFah+xTD7J+zqnuHr6IuMS3+s9t7dy4sbKRwlcxP4n2z4
         ZXjJbQpEEfOec36EF89By4ZHNHxLmDx/DZvPA5nv10emet1Omki9w2HF3hcaAijZDL2l
         jb1g==
ARC-Authentication-Results: i=1; mx.google.com;
       spf=softfail (google.com: domain of transitioning {sender}@{my-domain} does not designate {my-ip-address} as permitted sender) smtp.mailfrom={sender}@{my-domain}
Return-Path: <{sender}@{my-domain}>
Received: from {my-domain} ([{my-ip-address}])
        by mx.google.com with ESMTP id 141si8443110ile.78.2020.03.22.02.12.03
        for <{recipient}@gmail.com>;
        Sun, 22 Mar 2020 02:12:09 -0700 (PDT)
Received-SPF: softfail (google.com: domain of transitioning {sender}@{my-domain} does not designate {my-ip-address} as permitted sender) client-ip={my-ip-address};
Authentication-Results: mx.google.com;
       spf=softfail (google.com: domain of transitioning {sender}@{my-domain} does not designate {my-ip-address} as permitted sender) smtp.mailfrom={sender}@{my-domain}
Received: from {my-domain} (localhost [IPv6:::1]) by {my-domain} (Postfix) with SMTP id BEFC13C00DD for <{recipient}@gmail.com>; Sun, 22 Mar 2020 05:12:02 -0400 (EDT)
Content-Type: multipart/related; boundary="===============0513549325=="
MIME-Version: 1.0
Subject: Calvin and Hobbes of the day
From: {sender}@{my-domain}
To: {recipient}@gmail.com
Message-Id: <20200322091202.BEFC13C00DD@{my-domain}>
Date: Sun, 22 Mar 2020 05:12:02 -0400 (EDT)

--===============0513549325==
Content-Type: multipart/alternative; boundary="===============1139914447=="
MIME-Version: 1.0

--===============1139914447==
Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit

Calvin and Hobbes of the day email. Image not displayed for techical reasons. To unsubscribe, send an email to {sender}@{my-domain} with the subject "Unsubscribe please". You may briefly continue to recieve emails until the system processes your request.
--===============1139914447==
Content-Type: text/html; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit

<img src="cid:30cbc076bf26fa38547141665a3874fc0bdc91b550d5b9523fbf033cd64d0d8b"><p>This Calvin and Hobbes was originally published on Tuesday, March 22, 1994. Enjoy!</p><p style="font-family:serif;text-size:9pt;">To unsubscribe, send an email to <a href="mailto:{sender}@{my-domain}?subject=Unsubscribe please">{sender}@{my-domain}</a> with the subject "Unsubscribe please". You may briefly continue to recieve emails until the system processes your request.</p>
--===============1139914447==--
--===============0513549325==
Content-Type: image/gif
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-ID: <30cbc076bf26fa38547141665a3874fc0bdc91b550d5b9523fbf033cd64d0d8b>


--===============0513549325==--

I can provide most of my Python script if that would be helpful, and can try to obtain the source of a (working) email from another Gmail recipient.

EDIT: A few of my other recipients have also reported that this has happened in the past, but it only lasted a day or two, and they didn't think much of it. (I believe they are all also Gmail users...)