Object collision ignores landscape components that have been retopologized if the landscapes collision mip level is set to 1 instead of 0.
1. Open attached project
2. Select landscape
3. In Details Pane searchbar type Collision Mip
4. Set Collision Mip Level to 1
5. Press simulate, watch middle landscape component
Head over to the existing Questions & Answers thread and let us know what's up.