If i remove this line:
https://github.com/symfony/mime/blob/6.2/Encoder/QpContentEncoder.php#L45
the line break is rendered as expected

https://www.freesoft.org/CIE/RFC/1521/6.htm says

Rule #4 (Line Breaks): A line break in a text body, independent of
what its representation is following the canonical representation
of the data being encoded, must be represented by a (RFC 822) line
break, which is a CRLF sequence, in the Quoted-Printable encoding.
Since the canonical representation of types other than text do not
generally include the representation of line breaks, no hard line
breaks (i.e. line breaks that are intended to be meaningful and
to be displayed to the user) should occur in the quoted-printable
encoding of such types. Of course, occurrences of "=0D", "=0A",
"0A=0D" and "=0D=0A" will eventually be encountered.

So I guess generally that preg_replace seems correct?

The implementation in swiftmailer looked a bit different: https://github.com/swiftmailer/swiftmailer/blob/master/lib/classes/Swift/Encoder/QpEncoder.php#L279

Actually I cannot reproduce this issue.

I just tested it via

bin/console mailer:test my@email.com --body=$'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\naaaaaaaaaaaaaaaaaaaaaaaaa'

and the email I receive is

Hmm, that's strange.
Can you reproduce it with the setup i posted ?
The difference is the usage of file_get_contents() to get the email body.
I don't know if that makes a difference

I can also not reproduce this with your php snippet. The email arrives just fine.

Raw email:

So I don't think there is any issue on Symfony mailer

Hey, thanks for your report!
There has not been a lot of activity here for a while. Is this bug still relevant? Have you managed to find a workaround?