Here is the source code of the received message:
source.txt
One thing is different now - recently I had to change the path @imgDir/logo/logo.svg to @imgDir/logo/anonymne.svg

Not sure how the weird =s or line breaks happened, but other than those this is perfectly valid html to show the image inline and not as an attachment.

Here I uploaded the EML version. I opened it in Mozilla Thunderbird and here the attachment is now correctly not being shown. So, apparently only web-based email clients like Outlook and Gmail are not able to hide the attachment.

Then I compared the EML message (download) received from PHPMailer, which works even in web-based email clients. I found that message sent by PHPMailer has Content-Transfer-Encoding always set to base64, whilst Symfony Mailer sets only the inlined image to base64 and the rest is being set to quoted-printable.

Does anybody know how to change Content-Transfer-Encoding in Symfony Mailer?
I tried this, but it had no effect:

$email = (new TemplatedEmail())
       ->to(.....
 $email->getHeaders()
            ->addTextHeader('Content-Transfer-Encoding', 'base64');

It seems like it is not possible now: #16812.

I send all e-mails as synchronous.
I don't use BodyRendererInterface nor DkimSigner anywhere in the project.
I will take a look at your issue.