User blog comment:Yxz2014/Possible big plot hole??/@comment-9595671-20141023075243

I think the issue was that Eren hadn't been melted like the corpses in the titan vomit, and Eren was also still alive at the time. When Ymir ate Marcel, its possible she ate him in a way that instantly killed him and therefore prevented him from simply transforming inside her and escaping.

Basically I think a Titan only gains a Titan Shifter's powers when the shifter dies after they eat them, if the Shifter is still alive he/she still has an opportunity to escape.