It seems to be an old bug, see Asterisk (*) not seen while printing a survey form in pdf for a mandatory question! · Issue #1142 · kobotoolbox/enketo-express · GitHub and asterisk * not shown on printout · Issue #1152 · enketo/enketo · GitHub
@lognaturel, @martijnr , @Kal_Lam may know more, please.
Hint: There is also a difference in UI colour for the mandatory asterisk: blue in Collect versus red in Enketo. (See Colour in Enketo vs Android - #6 by wroos )
