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.