If there is a NULL byte in the string that is passed, quoted_printable_decode will crop everything after the NULL byte and the NULL byte itself.
<?php
$result = quoted_printable_decode("This is a\0 test.");
// $result === 'This is a'
?>
This is not a bug, but the intended behaviour and defined by RFC 2045 (see https://www.ietf.org/rfc/rfc2045.txt) in paragraph 2.7 and 2.8.