I only bother with the timeline for direct ancestors or persons of special interest. I used to create all these citations via Ancestry before FTM 2012, so the extra step of linking to the facts wasn't necessary--if done all up front. I think I must have noticed that sometimes it attached items to the timeline (and facts) and sometimes it didn't, but I never experimented to learn why.
I do find it curious that FTM in each case does see the media items attached to the facts via the source citation. I think this whole issue is because of an oversight on the part of the developers. Much of the way this works--whether through Ancestry or FTM--is inconsistent.