This may not be a fix for this exact issue but I had a related issue that took us hours to sort out.
When we built the project, a bunch of the new changes to maps (and content) were not being properly added in. Turns out the DefaultGame.ini file had the wrong map list, even though we were building with the correct map list. Apparently much of our new content was linked to those maps in some way that caused the build to make those meshes not show up in our packaged Release build.
Very odd indeed.
Anyway, check your DefaultGame.ini and that it has the correct maps populating the map list.
2019 Edit: Found my old post when I encountered this exact same issue (lol.) I deleted my Binaries, Intermediate, Build, and Saved folder – still didn’t fix it. Will edit this post with fix when I have resolved it.