I ported my character preset (heavily based on the “third person” starter project blueprint) into the open world demo collection. When I start the preview game, character falls through the ground, but still able stand on rocks. I didn’t changed anything in the demo level, except porting in character preset. I tried to move it high above the ground (in case it intersects with it), but it still falls right through it on the rocks.
Darth, I hit the same thing with the Open World demo content. I “fixed” it (although I have no idea if this is correct) by opening the “SM_CycRoom_01” mesh and changing it’s “Collision Complexity” to “Use Complex Collision as Simple”. This kept my character (and other objects, like a sphere I dropped into the scene) from falling through the floor on spawn.
I have the same issue in 4.12.4. As Chris Guidry said, opening up the mesh and changing “Collision Complexity” to “Use Complex Collision as Simple” was a workaround to stop falling through (however, the collision was bad and the players feet still go through). The thing I want to point out is that for me it was the “SM_Asset_Platform_02” mesh that had to be changed. Changing the “SM_CycRoom_01” mesh didn’t seem to have any effect.