I am currently experiencing the same thing.
This is a really bad problem/bug. I imported some EAPs from another model and they at first show up in design layers, but after navigating between different views they disappear and cannot be found. BUT, they do show up in section/elevation viewport. BUT promptly disappear when I enter the design layer of the viewport or select the new 2018 function of edit Section-In-Place.
I managed to have them show up briefly when I tried to delete the class they were on and merged them with the "none" class. But now they are gone again.