I did Randal's and Heath's special endings, get the Marcus's ending 3. I believe that it should be enough for Marcus's special ending, but when I try to do it, I do not have any answers highlighted in red as they was in both previous special endings. It's apparently some sort of bug, so I would be veru grateful if anyone could help me to get rid of it.

Sorry for mistakes. I'm not native in english. 

Hello! Are you playing as a Golgotha in Markus' route? Note that you also need to have seen Markus' normal ending (Amor Fati, with the unique CG) for the special ending to be unlocked.