This is related to bug #5491 but it is a different issue.

The description of MIME_QP_LONG_LINE reads:

The Quoted-Printable encoding REQUIRES that encoded lines be no more than 76 characters long. See page 18: [WWW] http://www.ietf.org/rfc/rfc1521.txt

However, the code in Mail/SpamAssassin/Plugin/MIMEEval.pm counts the 
decoded line length, as can be shown by adding some debugging code at
line 296:

      if ($where != 1 && $cte eq "quoted-printable" && ! /^SPAM: /) {
        if (length > 77) {
          open(my $debug_fh, '>>', '/var/tmp/spamassassin.' . time() . ".$$");
          print $debug_fh "long line: (", length, "): <$_>\n";
          $pms->{mime_qp_long_line} = 1;
        }

prints 

long line: (111): <Please open the attached document. This document was digitally sent to you using an HP Digital Sending device.
>
long line: (175): <To view this document you need to use the Adobe Acrobat Reader. For more information on the HP MFP Digital Sending Software or a free copy of the Acrobat reader please visit:
>

for the following message:

----------------------------------------------------------------------------
[...]
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable


Please open the attached document. This document was digitally sent to you =
using an HP Digital Sending device.


To view this document you need to use the Adobe Acrobat Reader. For more in=
formation on the HP MFP Digital Sending Software or a free copy of the Acro=
bat reader please visit:
[...]
----------------------------------------------------------------------------

The encoded lines are exactly 76 characters (excluding CRLF) long, but
MIME_QP_LONG_LINE is set anyway because it looks at the decoded length.