I restored to a course with a 2.4 questionnaire that contained a couple of questions with 'short label' and 'options' text. The separator between the 'short label' and 'options text' was the equal sign (=) rather than the new way of using two consecutive colons (:.
When I preview the questionnaire as an admin, both the short label and options text are displaying, with the equals signs.
When I added new questions to this questionnaire using (: the preview behaviour is as expected (short labels are not displaying).
Backup probably needs to detect the = and convert it to ::.