I spent today playing Diablo III on a different computer than normal, progressing on normal from early in Act III through most of Act IV. When I got back to my primary computer, I noticed I couldn't see all those cutscenes I unlocked on the other computer.
Here's a screenshot of the quest line:
And here's a screenshot of the available cutscenes:
On both machines I have been playing single player, if that makes any difference. I'll see what happens if I beat Act IV on my primary machine. Hopefully it unlocks all the cutscenes and everything's OK then.