I'm having the problem with Fane's romance dialogue as well. I replayed the game with a new character and went the same romance path hoping it would was just an issue with my character somehow, but the same bug occurred when I reached Act 4.
There are only a few things I can think of that I know for sure I have in common with both of my characters. One is I never asked Fane why he was ignorant about the world in Act 1. The question seemed unnecessary from an RP pov so I skipped it. I highly doubt that caused the issue though, since the majour romance scene before Arx still worked fine.
The other things in common are both my characters were male and they both used the method of persuading Fane to support them in the race for Divinity that didn't require an actual persuasion check. (Neither of my characters had Persuade leveled very high, so I didn't bother trying those options.) Can someone confirm that the bug still occurred for them even if they used one of the persuasion check dialogue lines? I'm tryna think of anything right now; I really want to get the dialogue to show up. x[
